With its rounded arms and large sofa pillows, Rowe’s Masquerade sectional creates a traditional look that works well with family favorites. (Courtesy of Rowe Furniture/Courtesy of Rowe Furniture)

About 25 years ago, Alexandria-based interior designer Jill Mastrostefano had what she described as "my number one 'aha' moment." Her critical insight: "I realized that everyone wants to come home and feel like they are on vacation.

"The feeling starts as they pull into their driveway. After an hour on the Beltway, people can't wait to get inside and immediately plop down in a relaxing space on a really comfortable chair," said Mastrostefano, the creative director of the P Four firm.

It took nearly two decades for Mastrostefano's idea to catch on, but things began to change about five years ago, as new houses shrank in size and households began to embrace an increasingly informal lifestyle with fewer belongings.

Although homeowners may not conceive of their home in these terms, the hugely popular, open-plan great room — a smaller, stripped-down, simplified version of the eat-in kitchen and family room with some personal touches but fewer pieces of furniture and knickknacks — often has the spare look and feel of a vacation cottage, albeit one with more upscale finishes and appliances than you would typically find in a vacation rental.

As the overall space in the great room has become smaller and the furnishings simpler, the windows have grown larger, and 8-foot-high sliding glass doors are increasingly common. All this additional glass area brings in more natural light and a stronger connection to the outdoors, another hallmark of the getting-away-from-it-all vacation house.

Choosing furniture for everyday vacation living, however, can be challenging because the way most homeowners use furniture in the sitting area of their great room and the way they sit in it have changed, Mastrostefano and several other interior designers and furniture designers said. The "off-the-clock" postures that have become the norm in many households — slouching, lounging, squirming, stretched out and napping, curled up and reading, using a laptop, or watching television — are not easily accommodated in the conventional sofas and armchairs that worked so well in your old family room or living room.

D.C. interior designer Skip Sroka characterizes the Kravet Malibu sectional sofa as “a style story” and notes that with the tufted back, you never need to fluff the back cushions. (Courtesy of Kravet/Courtesy of Kravet)

With squared corners, Rowe’s Martin sectional sofa has a more contemporary look. (Courtesy of Rowe Furniture/Courtesy of Rowe Furniture)

And functionally, sofas today are used for ever more household activities, often serving as a home office as well as a place for homework, relaxation and play. "In some families now, the sofa is used for everything, and it's become the hub of multitudes of activities," Phoenix architect and furniture designer Daniel Germani said.

With such a broad range of postures and activities, the perfect sofa solution for most households will be a sectional because it's unusually versatile. Unlike a conventional sofa, it's made up of individual pieces so that you can mix different sizes and include a chaise at one or both ends, and you can arrange the pieces into an L shape or a straight line. "It also has a great look and it adds to the casualness of the space," Greenwood Village, Colo., interior designer Lita Dirks said.

With so many variables, which sectional combination is the right one for you? The size of the area where you plan to put it is a good starting point, but for maximal comfort, the experts said the most important dimensions to factor in are the size of the adults in the household.

In most families they are not the same, and the sectional needs to accommodate everyone. To accomplish this, throw pillows — an integral part of a sectional and not just a visual accessory — can be added or removed.

Whether sitting or slouching, the adults should be able to have their feet on the floor and not dangling. "For most people this means a seat height that is about 18 inches off the floor," said Washington interior designer Skip Sroka.

"A seat depth of about 21 inches should be workable for most couples, but with this depth many women, who are typically shorter, will need to put one or two throw pillows behind them to get good lumbar support," Sroka said. "A taller person — someone over 6 feet 2 inches — may need a 23-inch depth, and this will allow the shorter person to curl up and read, work on a laptop or watch television. But for back support when simply sitting, the shorter person will likely want a sturdier pillow custom made to their precise requirements," Sroka said.

When a space is large enough, sectionals with unusual wavy shapes like Lazar’s Gemini model create a playful mood. (Courtesy of Lazar/Courtesy of Lazar)

The overall width, an important dimension because it's the one manufacturers use to distinguish one group of sectionals from another, should be about 36 inches, he added.

A 42-inch sectional — a size that is widely available — with a really deep 33-inch seat may look like what you want. But, characterizing it as "monstrous and awkward," Sroka said: "To get good back support you must wiggle backwards about a foot or else use a lot of pillows." A better solution for stretching out, he said, is to incorporate a chaise piece at one or both ends of a sectional.

In addition to the pillows, the back of the sectional itself also provides lumbar support. To be adequate, the back should project about 15 to 18 inches above the seat, Germani said. The overall height can be as much as 36 inches, but for households who entertain a lot, Germani designed a sectional with a 30-inch height (the same height as a bar stool) and a flat top that can be used for additional seating. It also appears to be a perfect perch for a small dog or cat to survey all the goings on in the big space.

For overall comfort, the fill and feel of a sectional is another important consideration, and this will depend on the amount of foam or down in the stuffing, the experts said. When it's 100 percent foam, the seat will feel very firm; if one of the adults has back issues, it will give more lumbar support, Sroka said.

When the stuffing is 100 percent down, you will feel like you're gently sinking into your sofa, Germani said. This works for some people, especially if they are young. But speaking for himself as a 51-year-old, he said, "It can be a challenge to get up, and an elderly person may need help."

For most people, "a foam cushion with a down wrap feels best and the combination looks better because it has a softer edge," Sroka said.

Armed with the relevant dimensions and fill preferences, how hard is it to select the right sectional?

It won't be hard to judge comfort, said Portland, Maine, furniture designer Doug Green, who recommended spending several afternoons in a furniture store trying out the floor models. "You can tell in five minutes if you feel comfortable with your feet on the floor and your back touching the cushion. If the stuffing is too soft, you'll feel really good at first, but you'll start to feel uncomfortable pretty quick."

But, Green added, "you'll need more time to see if it really works for you because no one sits still for very long. We are constantly slightly repositioning our bodies, so whether you're lounging or sitting, you need a sofa or armchair that lets you easily wiggle around. I always tell people to bring something to read and plan to spend at least 30 minutes in a piece of furniture before making their final choice."

Kravet’s Alhambra offers eight sectional pieces and numerous customizing options. (Courtesy of Kravet/Courtesy of Kravet)

Dirks said that for her, comfort was not enough.

"For pieces that you will be using for many hours every day, you also have to really like them," Dirks said. Her rule of thumb: "If you love it, buy it! When you surround yourself with pieces you love, the pieces will blend and go together nicely and your personality will come out because they're like bits and pieces of you. You may think you have no sense of design at all, but trust your instincts."

If you want to mix in a few pieces from your old house, Dirks said the love test should apply there, too. "If you love it, keep it. If you don't love it, get rid of it. People feel they must keep things that were more expensive, but they may not be the pieces that you loved or even sat in very often. It's like clothing — how many times have you bought an expensive jacket but never wore it? But a piece of jewelry cost 12 bucks and you just love it."

Compared to selecting a sofa, stools for the great room's central island may seem fairly straightforward. But refinements can affect your comfort here as well, Germani said.

"A wood stool with a seat that's molded to the curve of your leg and bottom will work, but if you want something that encourages the sitter to linger, get a padded seat." And, he added, "the stool will be more comfortable for anyone over 50 if you get one with a back."

Likewise with dining chairs — padded seats will be more comfortable and encourage the kind of lingering you want for a leisurely dinner party or special family occasion, Germani said.

Katherine Salant has an architecture degree from Harvard University. A native Washingtonian, she grew up in Fairfax County and now lives in Ann Arbor, Mich. If you have questions or would like to suggest topics for coverage, contact her at salanthousewatch@gmail.com or katherinesalant.com .