• If your codend is less than 2½ inches, your possession limit is 3,500 pounds per trip.
• If your codend is 2½ inches or more, but under 3 inches, your limit is 7,500 pounds.
• If your codend is 3 inches or more, your limit is 30,000 pounds.
• For vessels working in the northern shrimp fishery, your limit is a 1:1 ratio of shrimp and whiting, up to a maximum of 3,500 pounds of whiting.

Notes:

• The definition of "codend" depends on the size of your boat.  If your boat is 60 feet long or less, the codend is measured as the first 50 meshes (100 for square mesh) from the end of the net.  If your boat is over 60 feet long, the codend is measured as the first 100 meshes (200 for square mesh) from the end of the net.
• You can use a smaller mesh size than your codend in the rest of the net and still qualify for the higher trip limit.  For example, if your codend is 2½ inches but some of your net forward of the codend is 2 inches, you can still keep 7,500 pounds.  HOWEVER...
• You cannot carry spare 2" twine on board!  If you do, you're subject to the 3,500 pound trip limit (unless the spare twine is carried in patches less than 3' by 3').  Yes, you are reading this right: You can fish with 2 inch twine in the body of the net and still keep 7,500 pounds, but you cannot carry a bale of 2 inch mending twine on board -- if you do, your possession limit drops to 3,500 pounds.

These regulations are part of Amendment 12 and Framework 32.  And NMFS has a nice online summary of these rules as well.