I like taking photos and uploading them to Wikipedia Commons for free use. While I like to have images of just one object, I see one problem: You have no idea of the scale of the photographed object (see examples below).
One idea is to put a metre-stick next to it. The other idea is to put a reference object in the scene.
What would good reference objects be?
I thought of a 1-Euro coin, but many people might not know the size of it and it might be bad that it is flat.
Another idea was to put my hand in the scene. But this will probably not look good and people have very different hand sizes.