The closest antonyms for the word "sizes up" can be "underestimates" or "downplays". "Underestimating" something means to assign it a lower value or importance than it deserves, while "downplaying" something means to make it seem less significant or influential than it actually is. Therefore, if one "sizes up" a situation or person, they are trying to assess it or them comprehensively and accurately. In contrast, if one "underestimates" or "downplays" a situation or person, they are missing critical information and may underestimate the potential impact or significance of the situation, leading to potentially disastrous outcomes.