Hey! I'm looking at older mountain frames like late 80's early 90's Specialized Rockhopper/Hardrock/Stumpjumpers. Would like to convert one into a comfy, versatile commuter.

Anyway, whereas I understand framesizes for roadbikes (in centimeters and inches), I don't really understand what sizes roughly correspond to what heights for mountain frames.

In the following example, it says it's 26', which to me seems huge, yet it doesn't look it:
https://sfbay.craigslist.org/sby/bik...373068514.html

I realize it might actually be the same for both, but when seeing images of these bikes and frames, the sizes don't seem to match my eye for it.

So, my question is, for a 6' man with like a 30' inseam, what size MTB frame am I looking for?