I am pretty sure that at 5'8" you don't need to worry about being too short, unless things have changed dramatically since when I was in. I certainly remember chaps shorter than that - one Herc pilot I knew was known as "wee Jock" (guess where he came from). Don't think he was much over 5'6". One small thing you can check is colour blindness. A lot of people only discover they are colour blind when they have their first aviation medical. Affects about 10% of the male population, and it's a definite no-no for aircrew.