You can't make a decent human model with the Java3D API unless you are a mathematical genius and like doing things the hard way. Do your model in a 3D Modelling package and import it using one of the 3D file loaders out there.
Well i'm no expert on file loaders for Java but if your model vertices are held in Java containers or any other container like an Array then i would imagine that passing those model vertices through a scaling matrix modified by user input then this might give you the desired effect.
loading model in a container may be possible but container will not be able to get attributes and their values defined in 3D modeling package for the model.
This may be not a exact solution.
Well, have you looked at the code examples of 3D models out there used by Java 3D? I've seen a few myself. There must be something in this to get you started.