Max rim size for gz250?

[mithuth]

I searched and could only come up with tire size recommendations for the factory rims.
What I want to do is bob the rear of my 250 so I can run a 150/90-15 under a custom rear fender. And if possible, I'd like to sneak a 17 or 18" front wheel under the front fender (I will swap fenders or relocate the factory fender if necessary).
Also, on a 17 or 18" rim, what's the diameter equivalent of a 110/90-16 to fit said rim?

I've always been a fan of a larger front wheel with a skinny tire and a smaller rim with a fat tire on the back, but am a bit concerned it won't look good on a bobber/ratbike style setup.
Any suggestions or recommendations?