Where can I find information on a given pan's bottom diameter?

I am currently looking for a small pan. I measured the plate (ceramic) where it would be used (15 cm) and started looking for 15 cm pans online with little success. So far I’ve discovered that the advertised measurement for a pan typically refers to the upper diameter of the pan, not to that of the cooking surface, but that takes me back to square 1: how to know whether the pan will fit on the designed plate?

Where can I find this information, when it is missing from the product description?