you can easily get the extents of your model by setting @matrixoutput 1, and sending through a jit.3m.
you probably want to pass through a jit.pack 1 @jump 3 to only pass the first 3 planes (the vertex positions).
you probably also want to turn of normalization in the jit.gl.model by setting @normalize 0.
Thanx a lot for your interest in my project. I don't know if you understand what I want to do but I try to have a measurement out of a 3d model... I try what you tell me to do but with no luck.... Is it possible to have a little patch to get me started...