"Trying for size" refers to attempting to evaluate the suitability, fit, or appropriateness of something or someone, and in this context, it is challenging to think of antonyms that entirely oppose the phrase. Nonetheless, one antonym could be "ignoring the fit," whereby someone disregards the match or relevance of the item or person being considered. Another antonym could be "making hasty assumptions," where one draws conclusions without properly evaluating or examining the suitability of the object or person in question. Alternatively, "ignorance of the need" could serve as an antonym, where someone fails to recognize the necessity or relevance of evaluating the person or object's fit.