How tall do you have to be to play football in college?

It depends on a lot of things: position, level of competition (Division, conference?), etc. Height is more important for some positions than others. Height is pretty important for receivers, pocket QBs, DLine. It is an asset for most defensive positions, but not necessarily required. For most positions, there are examples of players who have been good despite not being as tall as most other good players at their position.

Height is not as important in college as in pro. Lots of teams run option offenses in college, which can be successful with very few tall guys (look at WVU over the past few years -- RB Slaton 5'9, QB White at 6', mostly small receivers).