Hi, out of my depth so don't bite my head off or feel offended if my contribution is way off the mark or far too simple, apologies if it is. I'm interested in answers to your question. I searched a little for "Length/Width ratio and capacitance in MOSFETS", and don't know if any of these may provide some relevant information if you skim-read them to the bits that mention W/L and so on:
Attachments
5.1 Device Structure and Physical Operation Electronics_Ch5.pdf