I think that to put object in pics, as Coops suggested is the way to go. Although I am not sure about matchbox, as they can vary in size.

I know it sounds stupid, but it is not always easy to imagine the size when you are given the measurement.

I have been recently looking for little buttons and came accross some 5mm, I looked at my steel measuring tape and 5 mm looked good.  Well, they are all but useless.  LOL

I learned the lesson, and put in my listings coin, depending on size of the item I am selling - 5c, 10c, or 20c.  Or having tape measure in the picture for large items, where size is important.

I also had a FB (thankfully with green dot) for something I sold, saying that although correct, it looks much smaller than the measurement it is listed  with.  ?:|