Seem to be all over the map. I size my mtbs by the top tube length, my Fly Ti effective TT length is close to my road bikes. That seems fine for me. In the last week I had one knowledgeable LBS dude say that approach is fine, same as your road bike or shorter, another LBS dude earlier this week said no, you get a TT that's 3cm longer than your road bike. I don't think that latter approach would work for me, the Fly Ti 57cm TT length is about 1 cm longer than my road bikes. But on the other hand I do have a 130mm stem on that bike. The bike feels great, I've been riding it for months, very comfortable with the geometry. I tried to picture that bike fitting with a 3cm longer TT yesterday. I don't think so.

What I seem to be gathering is that it's less cut and dried in MTB fit than it is in road bike fit. I'm getting the impression there's more of 'whatever you're comfortable with' - much more so than with road bikes. I'm talking XC racing MTB fit, I'm sure it starts varying with free ride, downhill, etc.

What about all that? I figured this would be the place to get some informed opinions on mtb fit.