My 5er came with ST205/75R15C on 5" wide rims. Barely adequate weight capacity for the GAWR of the trailer, and less than the GVWR of the trailer. After several blowouts, I went to ST225/75R15D. The bigger tires required 6" wide wheels, so I ordered t the new wheels from Southwest Wheel Company Steel and Aluminum Truck Wheels
. Everything fit fine, and that was the end of my blowout problems. One minor problem was that if the trailer hit a big bump or chug hole, the top of the trailer tire would rub on the underside of the wheel well. That was no big problem, and I put about 100,000 miles on that trailer with those bigger tires on it.
My current TT came with ST205/75R14C tires. Plenty of weight capacity for the GAWR of my TT, but I felt more comfortable towing with bigger tires. Not much choice for 14" trailer tires, but ST215/75R14C was available and had more weight capacity, so I replaced the stock tires with those. Plenty of room in my wheel wells for those tires, and they didn't require replacing the wheels.
My cargo trailer also came with ST205/75R15C. I replaced those with Maxxis ST 225/75R15E and 6" wide rims. No problems, and no more tire troubles on that trailer.
Another trailer is a 16' flatbed utility trailer. I replaced the ST205/75R15C and 5' wide rims with Maxxis ST2225/75R15E on 6" wide rims.
And yep, I have a 5x10' box trailer I use to haul a golf cart or handicap scooter or quad plus a generator and other stuff. Same story. ST205/75R15C replaced with ST225/75R15D. No problems.
: If you go to bigger trailer tires, be sure your trailer rims match the requirements for the new tires, else replace the wheels (with the correct hole pattern). And if you want to go from 15" to 16" tires, you may not find 16" rims to match the hole pattern of your trailer hubs.