26"
I've had terific results using a knobie with a moderate tread for my rear tire with a slick up front. The wider rear tire lends stability and balance when under load. Although I thought about purchasing a touring/road bike,I'm really pleased with the MTB touring configuration,since so many roads are in poor condition,plus I can go offroad with a MTB and roll out under the stars. I'm also sold on nylon slicks (1.5) and have passed the 1000 mile mark without any problems or blowouts.
I'm running Bontrager Mustang's on one MTB (bulletproof) and some Velocity Triple Vs (26" rims w/36 spokes) on the other. So far so good on the Vs,but it's too early to tell if they'll hold up under load.
Regards.