The word "sized up" means to assess or evaluate something or someone. Its opposite, or antonym, is to underestimate or undervalue. Underestimating someone can lead to missed opportunities or poor judgment in decision-making. It's important to remember that people and things can surprise us and should not be judged solely based on initial impressions. Another antonym for "sized up" is to overestimate, or to give something or someone more significance or importance than they deserve. Both underestimating and overestimating can lead to negative outcomes and risks, making it necessary to strike a balance in assessing situations and people.