With absolutely no other information or pictures provided, I'd go with the larger size any time IF it fits! The larger fender will stop more water, dirt and mud from slinging on the trailer and boat. But to verify that size you either need to provide more info or better yet, post some pictures for everybody to see what you're seeing... Of course this is all useless if you're talking about fenders for the boat itself...
I think the OP is talking about fenders for docking his boat .eaceful:
Biggest you can store. I would go with the 8's at a minimum.