I can’t tell you off the top of my head how large a large onion is. Or how hefty a medium tomato should be. That’s why it can be so maddening/frustrating/confusing when recipes call for a small, medium or large piece of produce and all you can do is stare at the supermarket bin and wonder.

“Small, medium, large. That’s clothing size,” says vegetable expert and cookbook author Deborah Madison. “Vegetables don’t come small, medium, large.”

While there are definitions using weights and diameters in “The Book of Yields” and “The Chef’s Book of Formulas, Yields and Sizes,” the typical home cook isn’t lugging a size chart down the produce aisle. And when it comes to cookbooks, one author’s medium might be another’s large, and so on. Medium might be the smallest you can find of certain vegetables anyway. Golf ball-size shallots, I’m looking at you.

Madison, bless her heart, is someone who makes the effort to shed a little light on what she means when she refers to sizes. Leaf through her book “The New Vegetarian Cooking for Everyone” and you’ll find with each vegetable’s info page, a discussion of average sizes and what she means when she calls for a particular size.

But not every cookbook author or other recipe source is as diligent, and not every recipe provides an equivalent volume or weight when specifying a particular size. What then?

Good news: Most of the time, “It’s really not going to make a big difference in your recipe,” Madison says.

A little more carrot in your minestrone, a little less onion in your fajitas — it’s okay. After all, one of the great things about savory cooking — as opposed to baking — is the ability to tweak a recipe without disastrous consequences. That being said, you have to use common sense. If you’re making baba ghanoush and you use a small eggplant instead of a big one, your results will not be the same.

(Tom McCorkle for The Washington Post; food styling by Amanda Soto/The Washington Post)

Smalls of anything seem particularly hard to find these days at the grocery store: onions like globes, massive bell peppers. Even digging through the bin is unlikely to yield much variation. That’s not by accident. “We aim to be as consistent as possible when it comes to our produce,” says Al Rivera, the director of produce and floral at Giant Food. “We aim to have the produce in each specific category of fruits and vegetables to be similar in size.”

Not all customers care about size, but Rivera says the company does see some patterns in purchasing. “We find that a family of four is usually more likely to look to buy larger size produce, while a family of two might be more inclined to buy smaller size,” says Rivera. “Though the science is not exact, the larger the family, more often the larger the produce bought.”

If you’re in search of small produce, either because of a recipe or the number of people in your house, your best bet is a farmers market. When I went to one downtown D.C. market recently, I found everything from very small onions and peppers to huge tomatoes and squash. One vendor’s eggplant selection alone consisted of tiny Fairy Tales, slender Asian and the traditional Italian, ranging in size from small to extra large (according to my own personal definitions, of course).

Even putting in the effort, your search for a small or medium head of cabbage, for example, may fail. In that case, try to be creative with leftovers. “It’s okay to eat the same vegetable more than one day,” Madison says. Just don’t let them languish in your produce drawer.

Or be like Madison, who these days often cooks without a recipe. Your dish, your way.

More than anything, she says, “I just want people to relax. … Recipes aren’t really necessarily set in stone.”

More from Voraciously:

Six cheap tools to keep your kitchen sparkling clean

Think you hate okra? These slime-cutting techniques will change your mind.

A guide to dried chile peppers — your secret flavor weapon in the kitchen