(I'm talking about 3.5 here.)
I've been DMing for a while now, and I've always taken for granted the blanket system of space-per-size: 5 ft. for Medium, 10 ft. for Large, 15 ft. for Huge, 20 ft. for Gargantuan, etc.
Only recently did I notice that, especially for Huge and Gargantuan...