I am looking to buy a road bike (specifically the one linked below) I am trying to figure out the sizing. So the road bike I am looking to buy is 21.5" and I am 5'11 so my bike height range should be anywhere between 56cm-60cm. This bike would technically be a little below my recommended ride height. Again this is going to sound really dumb, but couldn't I just raise the seat to make it the height that suites me best? Maybe road bikes are more involved than just the seat maybe the handle bar height is too small? No clue this is why I am posting this because I need good advice before I spend the money. Please and thank you very much!

https://tallahassee.craigslist.org/bik/5017445800.html

Yeah, that's a 54cm so yeah you could easily just adjust it to fit you. You might have to get a longer stem, but could possibly fit you pretty easily. I ride a 54cm and I'm an inch shorter than you.

Best thing you can do is go test ride it. See if just adjusting the seat will be fine for you.

It isn't just about the seat height, it has to do with the top tube length as well. As a fellow 5'11 1/2" er, I am on a 58. 54 will be very small for you more than likely. For me, even a 56 is pushing it on the small side.

Are you sure you got the sizing for a road bike correct? If you are taking that dimension from a mountain bike it is significantly different.
I'm quite a bit shorter than you and when I rode a road bike the size that fit me perfectly was 21". I doubt anyone 5' 11" tall could ride that bike comfortably.

Agree! I am 5'4" and my current bike is setup as an upright townie and has a road bike-like geometry. I got a 54cm and the top tube is 55.5 cm. The reach with the upright bars is perfect for me, and i am way shorter than you. On a road bike, I'd likely take a 52cm and my inseam is about 31.5/ pubic bone height is 80cm. My reach is about 74cm. That sounds tiny to me! You'll be very cramped in the cockpit.

If the top tube is really long it could work. But it would need to be an oddly proportioned bike with a 57cm+ top tube.

I'm 5'11" and my last road bike was a 56, and it was on the small side.