to estimate or assess someone or something

When two men are engaged in a heated argument and it is clear no resolution will be made, one male will say "size up", prompting both participants to expose their penises. The one with the larger penis wins the argument by default simply because his penis is larger. No exceptions.

when u measure each others cocks guys

The act of checking someone out to see if you can take them in a fight

The phrase "size up", usually used in arguments, essentially means to "stand up and fight"

The act of looking at someone and determining if you could take them in a fight.

Basketball term usually used to gauge one defenders ability