size up

act one's age, not one's shoe size

Synonym of act one's age (“to behave in a mature manner; to avoid being childish”).

cut down to size

To humble or humiliate, especially someone or something that is perceived as overly domineering or too proud.

pick on someone your own size

An admonition of cowardice, usually said to make someone cease harassing or bullying someone else, especially where the bullied party is physically smaller or weaker than the bully.

size queen

One who prefers sexual partners with large penises.

that's about the size of it

That is the situation; that is how it is.