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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.