Congrats on the milestone! 21 is a great age (I'm 25)!
I'm 5'8 and as a female that's considered to be tall -
Whether or not you're considered small or average would be dependant upon context. If you're in a European country with higher altitude (such as Holland) you may be considered small (en general). If you're in Asia - you'd be considered to be relatively tall!
I think you're about average. I would not say small...not that you were asking for this but in my opinion 5'7" - 5'10" is the perfect height range for a man! :) (although I can only speak for my preferences... :p)
I think 5'9'' is the national average for males around our age. I am 21, and the exact same height as you. Depending on where you live that may seem small or big, from where I am it is a little small, but I know friends who live elsewhere that are the same height but are giants.