It's just fine to think of huge families all gathered 'round to enjoy Mom's turkey and gravy, Sis' sweet potatoes with miniature marshmallows and lima bean casserole and Aunt Laverne's homemade mincemeat pie. That is Thanksgiving to a lot of people. They wouldn't give it up for the world.

But in the era of smaller households and upward as well as cross-country mobility, it's as practical as it is preferable to scale down the Thanksgiving celebration. A turkey is sometimes an albatross of leftovers; smaller meals with smaller birds accommodate the intimate gathering.

But the meal should be no less festive. A hen can take the place of a turkey with leftovers enough for salad the next day -- an appropriate meal to follow Thanksgiving. Quality stands in for quantity on the plate, which includes multicolored vegetables -- a trio of pure'es set off by the rich red of cranberry sauce.

The pure'es of broccoli and carrots are easy to make with a food processor or blender. For the appliance-less chef, contrasting piles of color and texture -- setting one against the other in a row like the Italian flag -- would be no less pleasing. Serve only the most beautiful broccoli florets, carrots in uniform sizes and potatoes that have been mashed through a ricer, food mill or electric mixer. Frozen winter squash comes already pure'ed and helps to cut corners without sacrificing color (substitute it for the carrots, if desired).

The cranberry recipe is traditional, easy and holds well so that the tradition carries easily through weeks of the holiday season without taking up a lot of room in the refrigerator.

It's hard to believe that a Thanksgiving meal can be simple. Yet the following dinner, suitable for four, can be created with one quick trip through the express lane of the supermarket, provided you have such staples as flour, sugar, salt, pepper and butter or oil already at home.

EXPRESS LANE LIST: roasting chicken, rosemary, potatoes, broccoli, carrots (or winter squash), milk, cranberries, oranges.

ROAST CHICKEN ROSEMARY

Roasting chickens are usually between 3 1/2 and 5 pounds and should cook to an internal temperature of about 180 degrees (about 15 or 20 minutes per pound). The bird will keep its shape better if it is trussed (consult an all-purpose cookbook) or at least has its legs tied together and wings tucked underneath the body. 4- to 5-pound chicken 5 tablespoons butter 2 teaspoons dried rosemary Flour, if desired Salt and pepper to taste

Heat the oven to 450 degrees. Cream butter and rosemary together and spread between the skin and the meat of the chicken over the breast, legs and thighs. Place the chicken, breast up, on a rack over a shallow roasting pan and place in the oven. Reduce heat immediately to 350 degrees and roast for about 15 minutes to the pound. Baste frequently with pan drippings. Serve with pan drippings or with gravy.

To make gravy, combine equal amounts of flour and butter or chicken fat from the pan (pour the drippings in a measuring cup; chicken juices will sink to the bottom and can replace part of the liquid, and fat will rise to the surface to be skimmed off and mixed with the flour). Use about 1 tablespoon each of fat and flour for 1 cup of liquid to make a fairly thick gravy. After whisking fat and flour together over moderate heat, stir in water or chicken broth gradually. The mixture will become very pasty at once as the flour cooks. Continue adding water gradually and keep whisking until all the liquid is added. Boil for several minutes and pour into sauceboat.

Note: Whole aromatic vegetables (celery, onion, carrots, leeks, turnips, etc.) may be added to the cavity of a chicken, if desired, in lieu of the absent stuffing.

PURE'ED VEGETABLE TRIO (4 servings) 5 potatoes 1 pound carrots Salt and pepper 1 bunch broccoli 9 tablespoons butter About 1 cup milk

Wash and peel potatoes. Place in a saucepan and cover with cold water. Cover and let simmer until fork-tender. Peel carrots, cut into 2-inch pieces. Place in saucepan, cover with water, season with salt and pepper and simmer until fork-tender. Wash broccoli and steam until fork-tender.

Let broccoli and carrots cool. With electric beater, mash 4 1/2 potatoes (reserving half of a potato for broccoli) with 3 tablespoons butter and enough hot milk (about 3/4 cup) to make a firm but smooth pure'e. Put broccoli and 1/2 potato in food processor or blender and chop with metal blade for about 30 seconds. Add 1/4 cup milk and 3 tablespoons butter and turn processor on and off until broccoli is pure'ed. Remove broccoli, clean processor and repeat with carrots. The amount of milk you will use, of course, depends on the vegetables you use.

With a large serving spoon mound pure'es next to one another on plate.

CRANBERRY SAUCE 1 pound cranberries 2 cups boiling water 2 cups sugar Grated rind of 1 orange

Rinse cranberries and pick over to remove soft or spoiled berries. Place them in a saucepan with boiling water. Return the water to a boil over high heat. Add sugar and orange rind and continue to boil until their skins burst, cooking a total of 5 to 10 minutes. Serve cold or at room temperature as a relish with poultry and game. Some people replace mayonnaise with cranberry sauce on turkey and chicken sandwiches.