I've been riding my hard tail with semi-slicks, which are a couple of inches wide. It weighs a lot more than my road bikes, but I'm not trying to keep up with anyone, so no worries. And those tires soak up everything on the crap roads around here.
Tire size and construction vary greatly. You really have to read up on frame compliance, wheel compliance, tire and inner tube compliance to get started. Then, you've got to ride various systems to make valid comparisons. Although my mtn bike slicks pumped to 65 psi are safe on road irregularities, they're really not all that comfortable. If I want comfort, I ride my Parlee, with Reynolds composite aero wheels, Michelin Pro3 tires and latex tubes pumped to 110 psi - that's comfort. Well, in a warp speed, road bikey way.
On the other hand, my Scott Addict with Mavic Ksyrium SL wheels and 28mm Continentals is never going to be comfortable due to the stiffness of the frame and the wheels.