The antonyms for the phrase "most sizeable" are numerous, and it would depend on the context in which the phrase is used. If referring to a physical object or structure, words such as "smallest," "tiny," or "miniature" could be considered antonyms. Similarly, "insignificant," "inconsequential," and "trivial" could be possible antonyms when referring to importance or significance. Other antonyms could include "weak," "scant," "scrawny," or "minuscule" when referring to strength or power. Ultimately, the choice of antonym would depend on the specific context in which the phrase is used.