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3.
when u measure each others cocks guys
''I'll measure urs if u measure mine'' That would be a size up.
by rw-lad August 13, 2008
 
1.
to estimate or assess someone or something
He did size up the potential destruction, but did not act decisively in the end.
by The Return of Light Joker May 25, 2009
 
2.
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.
Male 1: "Dude, you're a fucking dumbass, McDonalds is definitely better than Wendy's."

Male 2: "Um, are you fucking kidding me, Wendy's is 10 times better."

Male 1: "Size up"

Male 2: Alright, fine, McDonald's is better.
by VillanovaJC March 13, 2011