You can find good sales on a nice selection of boards at Tactics and Evo. My DS always seems to end up with Element or Santa Cruz boards and you can get something nice for $100 or less. If you want to buy in-store, try Zumiez.
https://www.tactics.com/completes
For sizing, it may be a personal preference especially if you're talking about a teen and not a little kid. My tiny DD rides a 43 inch longboard with a wide wheelbase. If in doubt, I would go with something middle of the road.