Sources: My favorite seed catalogs and notes form previous seasons. Look for varieties labeled 'compact', "dwarf" "Tiny" or "Small". Check out the link below for more info.
Vegetable Container Size Suggested Varieties
Bean, Snap 5 gal Bush types
Beans, Lima 5 gal  
Beets 5 gal Early Wonder
Broccoli 5 gal (1 plant) Italian Green
Brussel Sprouts 5 gal (1 plant) Evesham
Cabbage 5 gal (1 plant) Discovery
Cabbage, Chinese 5 gal (1 plant) Michili
Carrot 5 gal  (rectangular) Nelson, Danvers Half Long
Cress 5 gal  (rectangular) Watercress
Cucumber 5 gal  (rectangular) Little leaf
Eggplant 3 gal Calliope, Hansel, Gretel, Fairy Tale
Garlic (for greens) 10" deep Most varieties
Kale (Collards) 5 gal  Winterbor
Lettuce 5 gal  (rectangular) Most varieties
Okra 5 gal Millionaire, Cajun Delight
Onion 5 gal  (rectangular) Yellow and White Sweet Spanish
Peppers, Sweet 2 gal  (rectangular) Green-to-red Bells, Mini apple, Islander, Lipstick, Apple, Antohi Romanian, Carmen,
Peppers, Hot 2 gal  (rectangular) Peppino, Early Jalapeno, El Jeffe (also Ornamental Peppers)
Potatoes 30 gal All Types
Radish 5 gal  (rectangular) Most varieties
Spinach 5 gal  (rectangular) Most varieties
Squash 2 gal Baby Crrokneck, Small Zucchini
Swiss Chard 5 gal Bright Lights, Bionda Di Lyon, Magenta Sunset, Bright Yellow
Tomatoes 5 gal  (1 plant) Valley Girl, Orange Blossom, look for "Tiny" or "Small"
    Small: Sweet Olive, Smarty, Gold Nugget
Suggested Link: Texas A&M  Make your pots lighter and get more harvest: Ups-a-Daisey