Your problem wouldn't lie witht he wheels, but more with the tyres fitted to them. When upping wheel sizes you reduce the size of the tyre wall, so the overall diameter should in reality remain the same. Obviously there is other issues to contend with, but you could fit 19"s or 20"s if you relaly wanted and keep thing rubber band tyres on them. If you were mad that is.
Sounds like there's either a suspension of tyre size issue going on there.