A 17mm wide rim can take 25-50mm tires:
http://www.schwalbetires.com/tech_in...dimensions#rim

As for what will fit your frame/fork,you need to measure like fietsbob and tsl said. Also note that wider tires are also taller,so in addition to side-to-side clearance,you'll need to check for height at the top of your fork crown,seatstay bridge,and chainstay bridge. The parts circled in red here: