For example:
http://www.navweaps.com/Weapons/WNUS_Main.htm
Battleship Main Guns: Heavy or Large guns 9" (229mm) or larger
Cruiser / Destroyer Main Guns and Battleship / Cruiser Secondary Guns: Medium guns 8" (203mm) to 4" (102mm)
Smaller Caliber and Anti-Aircraft Guns: Small or secondary guns 3" (76mm) and smaller
This is not canon but just a rough guide I use.
Does anyone have a better guide?