Its not just seat and top tubes remember to measure the headtube length...'cause you don't want too much fork steerer showing, not only is aesthetically displeasing but so could your back become if your slumped (unless your time trialing). I'm 5.9.5 with 32 inseem i ride a 54cm (c-t) x 54cm tt, 11cm stem with a 12.5 headtube. Its like a nicely fitting glove and probabally too small for a 5'11er. I would get the 56.