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To observe and form an opinion about or estimation of someone or something. A noun or pronoun can be used between "size" and "up." I strolled around the arena sizing up the competition. Always size up a car in person before you agree to buy it. I sized you up the minute you walked in the room.
size someone or something up
to observe someone or something to get information; to check someone or something out. The comedian sized the audience up and decided not to use his new material. I like to size up a situation before I act.
Make an estimate, opinion, or judgment of, as in She sized up her opponent and decided to withdraw from the election. This usage transfers measuring the size of something to broader meaning. [Late 1800s]
To make an estimate, opinion, or judgment of someone or something: She sized up her opponent. He sized the location up.