Why wouldn't you be able to call it size? Equal size doesn't imply equality elsewhere in mathematics or elsewhere in the world (Jimmy and Paul are the same height, so they must be the same person?).
Two people of equal height aren't the same person, so it would be odd for an article to claim that the way to convert a person to a real number would be to simply measure their height, and even odder to call that the "size" of a person.
