Depends on your skill level. Generally, 6" bigger than you is a good for starters. Are you a beginner? If so, maybe try something a little loggier <----(is that even a word). Anyway, Once your skill level progresses you can shed some inches and go shorter. I'm 6' and have boards ranging from 5'6" - 6'6" When I was a grom I was hard headed and learned on a shortboard. You CAN learn on a shortboard but have patience.
I learned on a short board as well( 1st. Board was 5'10 round nose 2nd was 5'3). Butnow I Am currently a junior in hs and I am roughly 5'7 140 and I have a 5'10 and my everyday board is 5'8. It's all about prefernce also, I prefer shortness in length and a little wider width and like to keep it about 2 1/16 thick (although not as short as slaters new boards). I think it's more fun to ride a shorter board because you have more oppertunities on a wave if it's powerful enough which is A major facter on east coast. I think a 5'11 is a great size. Go for it