@starkatt I feel like the most ubiquitous things you could use at a couple different size scales are
- A AA battery, as you mentioned
- A CD jewel case (or just a CD)
- A lego brick or possibly a tic-tac (unsure how internationally common tic-tacs are)
- A USB connector (or a lightning connector, which has the added benefit of visible pads you can use kinda like a ruler)
- Or just use a ruler, but that's a bit less "obvious"
-F/R