Most of us are. There is no standard sizing for shrimp and no regulations setting guidelines as what different sizes are called.
Shrimp are sized by the count per pound but often labelled differently. One vendor may call 21-30 shrimp large while another might call them extra-large. The difference between large, extra-large and jumbo depends entirely on where you buy them.
While researching an upcoming article for the Fiery Foods & BBQ Supersite highlighting a “Winter Shrimp Fest” I found a lot of variations on sizing and names for shrimp. Another interesting thing is some fishmongers call them prawns while others say shrimp (although biologically different, they’re basically both the same: decapod crustaceans that have exoskeletons and 10 legs). Just to add to the confusion, in Australia and the United Kingdom shrimp are referred to as prawns while in the United States prawns are usually called shrimp.
Some of the standard sizes you’ll find are:
61-70, 51-60 Really small shrimp (about thumbnail size), often used for shrimp salad and spring rolls.
43-50, 36-42 Usually labeled medium or medium large. Good for pasta dishes, po’ boys, tostadas and tacos.
31-35, 26-30 Often labeled large. A good size shrimp that can be used for shrimp cocktail and standalone entrees.
21-25, 16-20 Frequently labeled jumbo or extra jumbo. These are good for grilled shrimp serving six per diner or as appetizers.
U10-U15 Called Colossal, Extra Colossal and Super Colossal. These are expensive but impressive looking. The U indicated on the colossal shrimp sizes stands for under that many shrimp per pound. Sometimes there are U-8 and U-6 available but you’ll need to mortgage your house to get them!