Total Noob here throws in his .02, so take the following with a grain of salt, especially since chelboed has already spoken not once but twice, and while I esteem his advice on all things MTB highly, I suspect he is pulling your leg a little, and if you take him at face value, you may risk danger to your middle leg...

I'm 5'-11 1/2" tall - when I stand up straight, which isn't often - and recently bought a 19" Specialized Stumpjumper. According to the spec sheet, the standover height in the 19" frame was right for me, and I bought it without benefit of a test drive. Now, having put some miles on the bike, I find that I'm stretched out quite a bit, and wonder if the top tube isn't a little long for me. I don't think dialling in the fit will be a problem, as I can make it up with a shorter stem and maybe riser bars instead of the straight. No biggy, just a few more dollars. I am totally stoked about the bike, and don't regret buying the 19", but maybe the 18" Stumpy would have been perfect. Now windbag will sum up:

People here all seem to say a little bit smaller frame is better than too big. I would suggest test riding a few, as the geometry between sizes and manufacturers will vary.