The tire size upgrade you want to ask is about the bolt hole pattern which typically depends upon the hubs on the trailer. Most smaller than 12" wheels are four bolt pattern. The 12's I had on are what is known at 5 on 4.5's. WHen I replaced the wheels with 13" I had to put on new fenders as well. When I did that I had to move the axle to center it under the fenders. My old friend's boat runs 12" tires and he drives only 45 mph and has had trouble with the load. IMHO it's too small a trailer and wheels for his load. He puts about half a ton of stuff in the boat when he leaves on a trip. I decided to go larger 'cuz I'm a worrier.

Hardly anything goes easy...eheheh.

If you stay with the same diameter but go wider on tires sometimes, but not always, you have to get wider rims. If you got up in diameter you may need new fenders but it all depends on your trailer.

I like larger diameter for increased load capacity, better highway speeds with less wear on the bearings, etc. but if you don't go too fast it's mebbe a moot point. You can get higher capacity IIRC with wider tires to some extent. See link for extra info.

Best of luck!