John426, Often mountain bike rims are narrow enough for a 1" tire, not that I'd recommend it. Generally anything 1.75"-2" is perfectly fine. I like dual pattern tires on my mountain bike, semi smooth center with knobs on the side and use 2" Conti Double Fighter Duals.

Full knobbies mean a couple of things to me. Real aggressive tread with wide voids for soft soil or tightly packed small knobs for general use. My sister has some of the latter and they're really not too power sucking on paved roads and work well on gravel paths.

Brad