In recipes, don't be confused by the "U + a number."  Simply remember that 'U' stands for 'under' and the number stands for the number of shrimp per pound.  A good cooking size for entrees and filling appetizers is U15, which means under 15 shrimp per pound (and pretty consistently delivers about 14 of these delicious crustaceans).  Just remember, the smalled the number, the bigger the shrimp.