+1 You need your bicycle inseam for bike sizing.
Google bike sizing, you will find a lot of guides out there with lots of different opinions unfortunately.
Is there a reason you are looking at mountain bikes? What type of surfaces will you be riding on? A road bike is great on paved trails. On unpaved trails, I prefer the older rigid mountain bikes, they are available out there cheap. My most recent addition is a 1994 Trek 7000. I am 53, 5-11 and my Trek 7000 has a 19.5 inch frame, while I ride 56cm frames for road bikes (about 22 inch frame).