Are Shrimp Sizes codified?
This was covered in one of the early episodes of Good Eats, Crustacean Nation.  From the transcript of the episode :


  As soon as they are off the boat, shrimp tails—called 'green shrimp' in the biz.—are sized or sorted into various weight counts. The higher the number, the smaller the shrimp. Sixties-seventies, for instance, have 60 to 70 tails per pound, which is pretty small. A pound of 26-30s contain at least 26 but not more than 30 tails. Twenty one-twenty fives are larger and 16-20s larger still. The biggest have a U in front of the number signifying that there are less than that number per pound. There are, for instance, 12 or less shrimp in a pound of U-12s.   
  
  Now, there are still a lot of shops out there selling on adjectives like jumbo, medium, colossal. Not only are these terms not standardized or controlled, it's tough to even find 2 fishmongers who will agree on them. So, if your market doesn't sell by the weight count feel free to quote Mr. Big who told James Bond in Live and Let Die, "Names is for tombstones, baby."   Of course, I guess those are tombstones, aren't they?