@Albuquerque,
I am trying to teach you math as simply as I can. In math, terms have definitions, we are using words to describe mathematical objects, but the words have to be specifically defined. In
non-mathematical contexts words can have several different meanings.
If I ask you this question "What is the size of Fred?". You can answer this in several ways... you could give me his height, or his weight, or his volume. If I ask you to line people up in order of size... there are several possible correct answers (someone may be heavier but shorter).
If I ask you "What is the size of Fred's family?"... you will answer with an integer telling me the
quantity of people in a collection of people you consider to be a "family".
There are two points here.
1. The size of a set has nothing to do with the size of a member in that set.
2. In mathematics, you need to define the terms you are using in an precise way. In philosophy, you can shift around the meanings of words without much thought.