For determining what size tire can fit on your particular rim, there is a chart about half way down this page: http://sheldonbrown.com/tire-sizing.html

It is harder to tell what you might have clearance for on your bike. For example, my hybrid rims can accept tires as wide as 44cm and those tires actually will clear the frame if you sit and spin the tires. However, the tires actually rubbed on my front fork, probably when turning, wearing off the paint.