A good friend is sending his youngest to the local university. He's coming from the east coast and won't be bringing a bike. I volunteered to find a decent mountain bike. I know from bikes, but am a little out of practice on sizing. The rider is 5'8" and is pretty aggressive. Bike will be used mostly around town, with the occasional off-road ride. I'm thinking somewhere around 16 inches. Does this sound right?

Sure.

For 5'8", he could be either a 17.5 or a 16 depending on how long his legs are.

FWIW, I am 5' 8", and I have good luck with Specialized 17-inch frames, Gary Fisher 17.5-inch frames, and, oddly enough, an 18-inch hybrid bike. 15-inch frames generally feel too small for me. I've ridden 16- and 18-inch Giant Trances, and could never make up my mind between those two sizes of Trance. I mostly ride trails. While I like to think I'm aggressive, I'm sure that I'm not when compared to a college-aged kid.

But you know what? Why not take the kid out shopping after he gets there? That's what I'd do. Take the kid shopping. Spend some time w/him. Etc.

And good on you for helping your friend's son out.

JG - that's the kind of info I'm looking for, relative differences between brands. Thanks

Used bikes will get really scarce when 5K new freshmen hit town. Father and son have given me the go ahead to find a bike. I've found a few nice ones but so far are too big or too small.

As far as spending time, I think we'll do the occasional ride/hike, meal, or range session.

13" and everyone is pleased.

well i'm 6 ft 2 and i've got a 15 inch frame i find that it's alot easier to throw around into corners while still being big enough to ride normally if he's an agressive rider a smaller frame would be better suited in my opinion maybe a 14 inch but that's just my view

Never listen to someone who ignores punctuation.