Typically, to "size up" someone means to form a judgment of a situation or assess something. However, on TikTok is has a completely different meaning. ... Now, because anyone can edit Urban Dictionary and add any definition they see fit it's possible that someone has simply made this up to be gross.

What does it mean when you size someone up?

To size up is to evaluate or critically assess something or someone. When a chess player sizes up her competition, she makes a judgment about what kind of player she might be facing.

What does it mean to size up sexually?

The "spirit" of what sizing up means has been left intact, but an Urban Dictionary definition of "sizing" up refers to the act of a man looking at a woman's thighs and guessing whether or not she could endure certain aspects of his anatomy for a pleasurable intercourse experience.