Now I want to make a modification to the loaded model. For example, I
want to change the height, or change the shoulder width, stomach size,
Is this possible ? If so, how do I go about it?
Yes, it is possible.
The "How" depends a lot on what exactly you're after. Altering height can be done simply by applying a scaling transform matrix before rendering. The other things you mentioned require more work, and there are several ways to go about it.
One way would be to make several versions of the mesh and interpolate between the desired features (like stomach size). Interpolating between two meshes is straightforward, but if you have several features you want to alter (like stomach size and hip width), it gets a bit more complicated. One way to solve this is to somehow "paint" weights to the vertices (through, for example, vertex colors in modelling programs) and use those when interpolating, in order to avoid the change of one thing affecting some other thing.
Or maybe you just want to apply some kind of "space modifier" to the mesh, where you just specify that "within this area, scale things outwards" or some such.
It all.. depends. =)