like 21 or 23" mt bike style would work. He doesn't care if it's used and will just be going to & from campus mostly (grad school). Any thoughts on how important it would be to get the right size? If I can't find a good used 23" would it be a big deal to find a 21" instead?

Mods-- guessing travel is right category since he'll travel/rec on a bike? Feel free to move if needed.