We bought hundreds of items on eBay back in the day, and had many times we opened a package and said, “I didn’t realize it was so small.” or “I didn’t realize it was that big.”
I see many attempts to solve the problem. Sellers include pictures of money, soda cans, and other common objects to provide something familiar that buyers can relate to.
It sort of works.
I didn’t want to use branded items for fear of trademark issues. Coke, for example, is really fast to realize possible unauthorized use.
I was photographing wall art, in particular, Lyman Byxbe etchings, and including an egg for scale worked well.
I could even use Pixelmator to put text on the egg image as a unique way to watermark my images.
Unfortunately, it didn’t work well when I switched to taking pictures of German nutcrackers and smokers.
I had fun fooling around with it though, and if your products are limited to flat objects, it would work well.