He should have enough room to stand up and turn around and that's it. Too big and they will pee on onside of the crate and sleep on the other. Lots of details with crate training, make sure you understand them all and don't forget to stick to a schedule that your dog can have success with. This site has some great info on housebreaking including crate sizes and how to get them use to the crate.
Housetraining Basics - Rules and Training Articles to Housebreak Your Dog