With regard to sizing I too am 5'11" and have a 56, a 55 and a 54 all of which are different geometry but all fit me. I also had a massive 60 frame that had riser bars so also fitted.
This whole sizing issue is probably the reason why I personally would never buy a bike online but prefer to go to a shop, sit on, have a test ride.
I know many people do buy online with no issues and I know when a new bike is purchased this way you can then experiment with stem length etc but for me that rather defeats the object of buying a bike that should be right from day one.
It seems the higher the price and more modern a bike the more critical a bike fit is. Many of the older bikes and cheap bikes have quill stems so offer a huge amount of adjustment. Often cheaper models are only available in two or even one size but rely on the quill stem to make adjustments. It's like folding bikes they can often fit anywhere from a 7 year old child to a 6'4" adult because they are designed to do so. High end models with threadless headsets, CF frames sometimes with integrated seat posts etc are much more critical for bike fit.
I've got an old Giant OCR with a threaded headset and quill stem, its a smallish frame but fits me fine. I probably couldn't ride the same bike with a threadless headset which I think was featured in the next year's model without buying stem height extenders etc adding a lot more weight and complexity. I think some OCR's before mine had a angle adjustable stem too although mine has a rigid quill stem.
I could be wrong but the OCR below preceded mine and it looks like it could be angle adjustable as well as height adjustable so you could quickly and easily adjust handlebar height and angle perhaps for different riders and different type of riding something that is lost with modern bikes.
Just making the point all bikes aren't equal when it comes to bike fit, some are fairly easy to get a good bike fit on and some are near enough impossible if you get the wrong size bike. Typically the lighter the bike more likely the more specific the rider's dimensions will be for that bike.
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