Most games for systems like the C64, CPC, Spectrum etc where originally released in large cardboard boxes, unless it was a budget title. It was only when a game had been out a while it would get re-released in a cassette jewel box. The dimensions of the big boxes vary too, but I’ve been working off 20cm x 15cm x 5cm which was the standard used for most. Something a bit like below.
This guys 3D scans site gives you a good idea of the wide variety of box sizes there are, even with the small collection of C64 games he has: https://bigboxcollection.com/#CollectionOverview>c64