Cannondale Jekyll tyres

Anyone know how big a tyre can be fitted in the back of a Jekyll? the front mech looks awfully close to the tyre witha 1.95 in. My brother wants to "toughen" the thing up a bit for place like Cwm Carn. Something like a 2.1 High Roller is what we are looking at. what about a 2.3 say on the front. Also do you know of any tyres that tend to come up hugely over wide for their nominal size . he doesn't want to buy say a 2.1 to find that its way too big.