I'm 6'1, usually ride road bikes at 60cm+ with a fist/fist and a half of seat tube showing. I'm looking to buy a used MTB and sizing agreement has me confused, mostly because whatever sizing chart I referred to suggested a 21" MTB, which translates to a 58-59cm frame.

My question is this: Do MTBs generally fit a little smaller than road bikes?

I am posting this in the Mechanics forum also.

Thanks in advance for any wisdom, an let me know if I failed to include any other useful information.

I'd say so, and many of them come in S/M/L/XL

I'm not a mountaineer, but I'd imagine the likelihood of The LowBoys getting intimate with a top tube is higher, so the MTB bike is made with a little more space in mind....

a 20 or a 21 will work. If the bike has sloping geometry use the imaginary level top tube to measure it. If you feel stretched on a 21, opt for the 20. That will get you in the ballpark. An even more apropos measurement is the distance from the ground to the top tube, just ahead of the tip of the saddle. That should give you at least 2 inches of freeboard (because if the hill gets too steep you'll need to stand down, and it is way farther to the ground than on the level).

Ask me how I know.

Yes, mountain bikes definitely size differently than road bikes. You need a shorter frame for nadular clearance and so the top tube isn't limiting the angle of the bike.

For a guy 6'1 a 21" frame is perfectly reasonable. The number I look for is the top tube length, it's a virtual number since the thing is sloped, but about a 23" top tube works for me. You're mileage will vary, but you see what I mean. The seatpost can be as long as it needs, since on an MTB you're out of the saddle for any hard pedaling.