It's not so much "can I fit them on the rim" as you can pretty much cram any 26" tire on a 26" rim... it's more what will fit in between the chainstays (near the bottom bracket) and for more fun and games - what will still let the chain clear w/o offsetting the wheel sprocket/engine.
I'm runnin' 26x2.125s with some room to spare - but be warned, not all tires are exactly what they claim and actual sizes vary, in particular side wall shape & bulge... so anything much more than 2.125 and it may take some experimenting
26x2.125s are a pretty nice, large tire & a good place to start;