Depends on what you are trying to duplicate? If it is a craw and you are dragging the bottom then use the same trailer as your other jigs. Most of the time I am fishing a swim jig I am trying to imitate some sort of other fish. For open water a 4" grub works but most of the time I am fishing in or over shallow water grass. My top 3 are; Keitech 3.8" Fat impact, 4" Pline Kick-R minnow, and Damiki Anchovy shad all of which are boot tail minnows. Other baits work but I found 4" is the perfect size as 3.5" is to short and the skirt interferes with the trailer and the 4-5" is too long.
Allen