You've seen enough pictures of people holding them to get a good reference for the size.
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).
Let me Google that for you: https://images.app.goo.gl/hW7G3NaixUd4sNUc7