Size up Definition 1 of 3Pro Tip 1/3Quick Judgement Used when someone makes a fast assessment of a situation or person. The detective sized up the crime scene in seconds. Pro Tip 2/3Initial Impression Reflects forming a first impression, which may not be accurate. I sized him up as trustworthy, but I could be wrong. Pro Tip 3/3Subtle Analysis Implies a nuanced read of nuances that aren't overtly visible. He sized up the deal and found potential issues.