Mountain bike sizing - cm / inches / sm/med/lrg -what's what?

fat bloke wrote: »

I get road bike sizing, but mtb sizing escapes my tiny, cholesterol-short-circuited brain. I know there are so many different mtb models and geometries but is there a basic rule of thumb?

Like what's a 49cm mtb??? -Can I translate that to inches? Is it any way comparable to a 49cm road bike frame (which would be tiny).

No, there isn't really a rule of thumb. A medium can vary wildly across different brands, - in size, feel and riding et experience.

Mountain bikes can vary on nearly every geometry measurement (top tube, reach, stand-over, bottom bracket height etc. etc.), -then you have the angles.
Reach and effective top-tube are generally the most important.
Many have a sizing chart, - which will more or less see you right, - though some manufacturers are notorious for "coming up small" (eg Canyon, Santa Cruz, though they are changing) etc.
Also, it can be difficult even to compare against bikes a few years old, - as very newer bikes tend to be longer and lower.