size up - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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]