if you do not want to measure everything yourself - best bet is to find the catalogue.
Aside from seat tube length (which is pretty meaningless on MTB with modern sloping TT) - there are reach, stack, effective TT, standover height (and SO height can be affected by how tall are your tires). Moreover - if you get the exact same bike which was fitted for someone else it may have longer/shorter stem, stem angle could be different, your bars may have additional rise, they also may have setback and may have different setback angle.
Look at the pictures of frame geometries from different manufacturers (I recommend Surly, they have very detailed sizing info) and replicate the measurements on your bike.
here is an example to get you started,
Karate Monkey 27.5+ | Bikes | Surly Bikes
