20" rims possible on xB?

 

Quite possible. Definately been covered. Requires some monor modifications for most 20" wheels to fit.

 

how "minor"?

 

It all depends on the actual wheels specs. In most cases when you buy a 20" wheel that was designed to go on a small front wheel drive car you just have to do a little trimming or "massaging" of the rear suspension and wheel well/fender lip.