I started out with a small case and a collection of small modules. If I had my time again, I would quite possibly buy a small case, but I wouldn't get the same modules. I thought having more smaller modules would mean more variety, but in the end I found it frustrating and a little unfulfilling. If I could start again, I think my first modules for an 84HP case would be:

A Wogglebug, or equivalent
A Maths, or equivalent
A DPO, or equivalent
An Optomix, or equivalent
A Mimeophon, or equivalent

I feel that those modules could keep me happy for months, if not years, and would be a good basis to build up to a larger modular system.

https://cdn.modulargrid.net/img/racks/modulargrid_1453324.jpg