How to quantize the shape of a molecule?
There are lots of descriptors of molecular size and shape that come to mind:


Moments of inertia (e.g., how different are they)
Radii (e.g., enclosing the molecule in a best-fit box)
Surface area or polar surface area (there are existing group-based methods)
Volume (e.g., of the van der Waals surface)


Some people have also used 3D shape descriptors like "spectrophores".