Depending on the design, also check to make sure the tires will clear your brakes. You'll find this out when you go to put them on, but I would ask first before buying them.

I have a large front tire on my mountainbike and getting it on and off is a pain because the tire doesn't clear the brake even when it is open all the way, so I have to jam it through the pads, which causes the pad to twist. I have Deore XT brakes that don't swing out of the way as far as the old cantilevers, so you may not have this problem depending on your brake design.

Other than that, they should fit the rims fine. I think you could use up to a 2.25, but unless you are doing serious off roading, I wouldn't bother; the rolling resistance goes way up with the bigger tires.

I remember taking off my off road tires and putting on more of a street tire when I converted a mountain bike to a touring bike, but I don't remember the size-probably a 1.75.