Taken from Good Housekeeping -- February, 2004
Color and texture - Raw shrimp should look firm and have shiny shells. Avoid those with black spots (a sign of age) and yellowing, sandpaper-like shells (an indication of mishandling). Also pass on shrimp that have a stale ammonia or fishy smell.
Peeled or unpeeled - Shrimp best retain their flavor in the shell. But if you dread deveining and peeling or are short on time and want to buy them cooked, make sure they're firm to the touch and no long translucent.
Which size - Large shrimp are the choice for pan sautes or grilling on skewers; medium ones work well whole in tosses or pasta dishes; the small ones are perfect for rice and shrimp casseroles, chowders and soups and chopped up in a salad. When you are following a recipe that calls for shrimp by size, don't rely on the store's designations or large, medium or small. There is no industry standard for shrimp sizing, so choose the size that will best suit your recipe. The smaller the shrimp are, the less expensive they'll be but you'll spend more time peeling.