Mountain Biking - Geometry of the frame and body type ?

I did some driving and found a shop with a 2006 Gary Fisher Marlin for $419. ( Would this be a good deal. It's $120 off the list price. ) It has what I want, as far as hardware, I think. The question I have is...How do you know if a given geometry ( Such as this Genesis frame ) will be a good fit. All the bike brands have a diagram of their frames. But..what do you look for? If I am one of those guys who needs a long sized shirt, meaning my torso is sort of taller, would the Genesis, which seems to be more "laid out" be a good fit ?

I know some of the responses will say, "Go ride it ! " But my retort is that , I won't be able to ride it for more than a few minutes. Even if they set it up right, is that REALLY long enough to know ?

admittedly, it's always hard, as a 1st timer, to really know what's good for you or not in terms of geometry on the bike. but speaking GENERALLY, I'd say you probably need a bike that has a longer cockpit, or a higher proportion of cockpit length to seat-to-crank length, for hardtails this should come about in bikes with steeper seattube angles and shorter chainstays/seatstays. dont know about the marlin but i think GFs are built with shorter chainstays so they might be right for you. otherwise, you can get a longer stem, but tends to make steering a little weirder.

Don't give up on the "Go ride it!" philosophy - suggest that when you try it, setup the SEAT HEIGHT to your comfortable leg-crank position (i.e. just slightly bent at full extension) and that will give you an indication of how comfortable the cockpit itself feels (i.e. sitting too high, too close/far from handlebar etc).

The Genisis geometry is great for tall riders - my son is 6'3" & rides a Marlin (after he destroyed his Big Sur). He very much prefers the GF feel to other frames. One of my brothers - same height, also swears by GF. Ride the bike - even if you spend 1/2 hour riding around a parkinglot!

I'm by no means tall, at 5'8" but proportionally, my torso is longer. I keep needing to tuck my shirts back in.

I just always wondered if there were some body measurments that would correspond with the distances in the diagrams of the frames that you could use to HELP determine what bike might be the best fit. You, someting like shoulders to hands, and hips to shoulders, or hips to foot, or .... ? That sort of thing.

Perhaps someone else has seen somethin like that, but I haven't. But the genesis geometry seems to reduce back strain, so if it's just your legs that are short, you should definately check the Marlin out.