is there a site or thread somewhere that tells ou the right size (small, med. / 17", 19" etc.) bike per the riders height??? I'm 6'0" and my girlfriend is 5'6" what would be a recomended starting point for figuring out what is the right size for us??? sorry for the totally noob question. but i'm almost not sure where to even start I know about what size (toptube ") for bmx bikes but mountains seem to be an entirely different animal to me right now.
There's no app for this.
the Top Tube is the key still. If you can nail that, then the frame size falls into place.
Check out Sheldonbrown.com (bike fit area) and also Colorado Cyclist bike fit area, both are very good.
In a nutshell, seated, you should feel comfortable; not too upright, not too far forward. Take your time, nail the fit, it's the key.
There are also manufacturers that offer a general guideline to sizing bikes.
Like for example companies like Yeti or K2 or Transition will say that if you fall into a certain height range, then a particular size would fit you.
Obviously a test ride will either confirm that it feels good for you or if it doesn't feel good for you.
Just go to the various websites and see if they have a sizing link for a particular model or range of bikes that you are interested in.