i agree.. it depends what class you want to run in, they normally have wheel size limits and control tyre

i think 205/50 on 15x6 or 7 is a good size that isnt too hard to make fit with flares or guard mods on a 1200
it gives you plenty of options for brakes and tyres aswell.

if you want to go wider you probably dont want to do it with 15's becuase wider also means a lot higher in profile aswell..
going wider with a decent rolling diameter may be possible on 13's but then you have less choice for brakes

its all a balance between what you want and what the rules allow you to do