I've owned one. But that was a million years ago. It's somewhere between the size of a box of matches and my current phone, but that's a pretty big range.
Edit: OK so maybe the reference of a gen 1 ipod is actually quite useful to me. But it's a 20 year old reference to use. Surely there are plenty of things around that are more useful as a reference point if you need one (or just provide measurements).