I'm looking into building an Xtracycle for family utility. My g/f is about 5'2", her mom is 5' and I'm 5'10". It's a stretch to consider fitting it for mom, but what sizes should I look for in a vintage steel frame MTB that would fit my g/f and myself? I've only wrenched and built vintage road/touring bikes thus far, so I don't really know how MTB sizing works. I've got my eye on a couple Bridgestone XO-2's in the area, both are "small". One is listed thus:
15" c2c height
16" c2t height
20" c2c length

cheers,
-Michael