Size Question

How tall are you? What's your inseam or when you buy jeans, what leg length do you get? Without further information, I'd guess you're a medium size frame or a 17"/18".

IMHO, you should be riding a "medium" frame. In a classic frame, this would be a 17" or 18" frame. With sloping top tubes and variations in design, this may vary, just like road bikes but the manufacturer should still call it a "medium". My son is 6' and buys 30" jeans, he rides a 05 Rockhopper that has a 17" frame. At your size, I would not try a large frame. Mountain bikes get thrown around during the ride something that never happens to a road bike, and you have to be able to clear the top tube for the times when you'll fall (it's gonna happen).

Longer torso, shorter legs. probably have arms proportional to legs.

I agree with the above, you probably need a medium size, or something in the 17 - 18.5" range. You can (and absolutely should) tune the reach with various stem and handlebar combinations, but watch your standover on older frames which usually have a straight top tube.

i ride a 20" frame. im 5`11" and a 32 inseam.
going to try an 18" this season....will see if i end up swapping frames

I'm 5' 11",, 32" Inseam and I ride a large full suspension Scott, I think Its a 19.5" frame..

Sounds like a medium frame will fit well. I would definitely recommend going down to your local bike store and try a medium of a similar brand to get a feel for it first. Just get them to do a service in exchange 🙂