Adjust your seat height and the world is your playground.
That does not account for different toptube lengths...
Yeah well I would rather ride a smaller frame with lots of clearance, and have it slightly shorter than a larger frame with little clearance and therefore nutting myself all the time.
Freeride and Downhill, you're always better off with smaller frames.
I'm 6ft and 235lbs on a med 17.5 Gary Fisher. I should have been sold a large but the sale was more important to the local shop owner! I have since Nov 08 replaced or repaired the cassette bearings, seat, seat post,(2) set of handle bars and just last week the chain! Size does matter in relation to the beating the components will go through. A couple of months ago I rode the new Roscoe 1 in a large, "it was a dream". Afterwards the factory rep saw my 17.5 frame and said "Dude you're on the wrong size frame!" I was so pissed off I could have gone Postal on the "Butt Monkey" who sold it to me and said it was fine. Check the site of the bike your interested in purchasing and they usually have a sizing chart to help you .
I would advise a test ride. Yeah, the dimensions might sound okay, but until you actually sit on it you'll never know for sure. I am 6'1" and I thought a large frame would fit me. But when I actually went to my LBS and checked all their bikes out, I found that a medium fit me better. I needed a shorter standover height because I have shorter legs. Everyone has a different body type, you just need to find the right bike for your type.