One thing that I've noticed is when looking around at headphones, the dimensions of the earcups aren't always readily available. This information could be useful when comparing headphones.

I'll start with the ones I have around. It's useful to have both inner and outer diameter measurements so you can see if they'll be ear crushers (supra-aural) or not.

Sennheiser HD-25: 7cm outer, 3cm inner
Beyerdynamic DJX-1: 9cm outer, 6cm inner

For oval cups, please measure along both major and minor axes.