If you are thinking of preparing some delicious shrimp recipe, it's important to choose the appropiate size for what you have in mind. Usually the larger shrimp are used in cooked dishes where presentation is important, and smaller shrimp will typically be used in ceviches, pastas, soups and a variety of dishes.
Here are some basic guidelines:
LARGE SHRIMP:
U10, U15, 16-20 or any large shrimp will cook great on the grill, baked recipes and also boiled. Salads and pastas will also look amazing with big shrimp but they can have basically any size.
SMALL TO MED SHRIMP
The smaller sizes are usually used for raw dishes lilke aguachile, or ceviche, poke, tacos and a huge variety of dishes.