I'm needing new tires on my trailer for the 24' Sea Ray. The boat weighs probably close to 5000 lbs. It has 6 lug axles on it, BUT the original wheels are 14.5's. I have a set of 15" trailer rims that I bought for it a few months back. So here are my current scenarios.
1. use the 15" rims and get 15" trailer tires ( I have priced the Nankang Freestar St 225/75R15 tires E load range for $56 ea)
2. Go to 16" rims and tires and sell the 15" rims. I found a set of five 16" trailer rims on CL for $125.00
2a. 16" tires. LT tires vs ST tires I currently have Milestar Grantland LT215/85R16 E load range on my dually for the last year. I probably have 6000 miles on them and have had no problems and was thinking of using the same size for the trailer ($88.00 ea)
2b. Or Milestar Grantland LT 225/75R16 Load range E for $84.00 ea

15 inch rims for a 5000 lb boat is fine. E load rnge is overkill but sounds good. 16 inch you may run into interference w fenders etc....

I have to agree, you are more than fine with 15" E load range tires

The 15" are more then fine. Load range E 225/75R15 have a carrying capacity of 2860lbs each x4 = 11,440lbs of carrying capacity at max inflation. You could almost carry the boat and trailer on one axle.

IF it were me, I would go with the 15" wheels and go with Maxxis E-rated ST225/75R15 tires. Those are the ONLY trailer tires I would ever consider running. I have them on three trailer currently, and no current complaints. My buddy just installed them under his 30' Outlaw too.

The Maxxis tires are not cheap, but IMHO they absolutely worth the price.