Antonyms for the phrase "most decent-sized" would include "least small," "tiny," "minuscule," "minute," "microscopic," "negligible," "insignificant," "inconsequential," "inconsiderable," "diminutive," and "small-scale." Essentially, these words all convey the idea that something is not large, significant, or important in size or scale. When seeking to convey the opposite of "most decent-sized," using any of these antonyms would help to emphasize a lack of size or importance in the described object or situation. It is important to choose antonyms carefully, as they can significantly impact the tone and meaning of any description or piece of writing.