If it an older mtb the sizing is going to be very close to that of a comparable road bike... my 87 and 88 Kuwaharas are 55cm and my road bike is 55cm and the fit on all of them is great... a 26 by 2.0 has about he same diameter as the 700:20 tyres on my road bike so standover is also very close.

One Kuwie has drop bars (and a shorter stem) while the other runs trekking bars.