I'm also 5'11, I ride Large frames o.0 I've been professionally fitted twice, both times they looked at me, put me on a medium. On both occasions the medium felt cramped and didn't sit well,
On a large frame I fit very well, I never feel over stretched or bunched up, on my road and mountain bike.
I could never imagine riding a bike with a small frame seriously 0.o
That being said, my Flatmate is 5'10.5 and rides a medium, he looks natural on a medium frame, and really really awkward and stretched and unstable on a large.
so I guess at 5'11 a medium is reasonable too. Just something I can't do....
Nice to see you on a new bike though
Years ago we would stand with the top tube under our crotch. On road bikes it was actually pretty easy. They say always ride the smallest frame you can. On the small I was really not over the pedals. I was behind em.
It seems spin positioning is the true key here. There is a perfect place to be over the pedals I think. Truly every bike design is different. It's the geometry, as some bikes just have long top tubes. Plus every person is different as far as back measurements, leg measurements.
You should ride what your comfortable with. The fitters are using a cookie cutter science and the variables are more than that. My Specialized FSR Pro in medium fits best. After years I'd have to say my Lemond is too big. That's the strange part is that you can come to conclusions even after a year or two?
There are rules here. Still if you were riding PCH and this super fast guy passed you on a way, way too small of frame there was always that question. Weight to power ratio?
Would the guy be that fast on anything?
Answer number two is maybe the correct answer.