Did a quick search. Found the state police inspection sheet. Look in section D. States brakes on all wheels if 5500 pounds. I haven't found the actual law.
I also found several rv and boat sites that state brakes are required over 3000 pounds.
As far as the tire size, I would go with the 15". Keep in mind that the actual trailer deck height is only 1" higher with a 2" taller tire. I have had my share of trailer tire issues over the years. Personally I don't like radials because of the sway. They are also visually harder to determine if they are low on tire pressure when doing a simple walk around inspection during fuel stops. I never get anthing but "D" rated bias now.
And like the other guys said, larger tire equals slower revolutions and cooler bearings.
just my .02