So for guard skills, height pretty much affects OD, JR, HA. JS is independent of height and driving/passing are affected very little. OD you need for both guard positions, JR you would need more for a SG, HA more for a PG. Still, that's really only two or three out of the six guard skills that his height would affect in any way. Obviously that's oversimplifying things somewhat, and of course you would like your SG to be able to handle the ball and you would like your PG to be able to shoot. But still, being 6'3" definitely doesn't make him worthless as a trainee. I'd say anyone 6'4" or lower has acceptable guard height. 6'5" or higher is pushing it, but it still can be done.