If you're an average sized rider, 28s are probably plenty. I weigh 240, and I do most of my riding on 32s or 35s (which don't really measure that big--Panaracer Pasela 35s are about 30mm wide).
How about fit, though? The rims will work with just about anything you can squeeze on, but if you have sidepull brakes (I don't know what's on that bike), you'll probably have clearance problems. If it has cantis, you're probably OK.