size up verb 2size-up noun 1forms 4 verb2 •To evaluate; to estimate or anticipate the magnitude, difficulty, or strength of something.It's a good idea for boxers to size up their opponents before their matches.Before we can begin to size up the problem, we'll need more information. •To wear a larger size of clothing.This shirt runs tight, so you should size up. size-up noun1 •The initial evaluation of an incident and how to deal with it. formssized up · sizing up · sizes up · size-ups