The size of the icon is measured by its width. It is a rectangle. Example: 20x20. The minimum size in some of the styles is 20, so you can't go smaller. But the maximum size is taken from the maximum icon width of the option you set.

I do not have time at the moment, but I'll try to create an example later today (evening on my part of the world). But I can't promise anything.

Note that styles don't need to follow special guidelines. This means that a technique that works with one style, might not work with another. This in turn means that it would be a good idea to create a custom menu style, which you can also do with the proxystyle