All I can tell you is wider is better.

I ride Schwalbe Marathon Dureme 42 c on my GT Eightball and I haven't noticed a real loss in speed.

I actually replaced Schwalbe Kojak 35 c slicks with wider threaded tires so I could go off pavement with stability and comfort.

If you're looking for a gravel road/adventure bike, you'll be wishing for high volume, fast-rolling tires for a comfortable ride.