Widest tire on a 23c rim?
See for a definitive (imho conservative) answer the almighty Sheldon Brown(bottom of page). The rim width can vary a little between wheels.

Having said that, I have been running 700-35c Cyclocross knobby tires on standard Shimano Ultegra road racing wheels for years without any issue.

Another really good tire for commuting and light offroading (gravel) to get on a CX bike is the Schwalbe Delta Cruiser (be sure to get the 700x28c version). Almost as fast as road tire, much higher comfort, long lasting and decent puncture protection.