Im 5'10" & I just bought a 16" Salsa Bandito (Scandium tubed HT) & it fits me really well for XC stuff.
I've also got a 13" Identiti Mr Hyde dirt jump bike which feels a good size for what its designed for but its not very nice to ride long distances on.
On my Giant Rainier - 17" (TopTube length = 22.64").
On my new Titus Racer-X - Medium, which is 19" (TopTube length = 23.25").
Their standover heights are both around 29.75"
I'm 5'8" which seems to be a height where I can go either small or medium frame on most bikes.
The fit on Rainier gives me a little more of upright position than the Titus. On Titus, a smaller frame might have been more appropriate for me as I am at the bottom range for a medium, 5'8" - 5'11", but I found a great deal on a medium frame and I couldn't pass it up.
13.5 and really i should have mroe clearence with the bar than i have, but this bike fit better than any of the other bieks i could get in my price range and i am used to being carful getting on and off the bike now, i am 4'11 and it took me several months to find my bike because of my hight
my bike: 18in 2004 specialized stumpjumper comp hardtail.
I could have gone larger i guess, but i like the clearance between MY vital components and the top tube - I'm a bit of a nutter off road which can introduce some pain in places its not welcome. Not that I'm suggesting I have areas where it is welcome. Errrmmm.....
I'm 6' and ride a large-size bike. When I bought it, I thought I'd probably be more comfortable on a smaller frame, but a pro fitter told me this was best for my measurements. I'm doing pretty well on it so I guess he was right.