I sometimes wonder how much the “size of Belgium” is used in non-English languages. Here’s a page from a German-Spanish dictionary, about the German phrase “so gross wie” (which means “the size of”).
There is a list of example of the phrase in normal usage, and the very first example is …
“Das ist eine Fläche so groß wie Belgien und die Niederlande zusammengenommen.”
Or in English .. “That is an area the size of Belgium and the Netherlands combined.”.