Antonyms for "decent sized" could include words such as "tiny," "small," "minuscule," "insignificant," "microscopic," or "diminutive." These words indicate a size that is below average or expected, and often imply that the object or subject being described is of little importance or value. On the other hand, words such as "massive," "gigantic," "enormous," "titanic," or "monumental" would be antonyms for "decent sized" that suggest a significant or noteworthy size. These words indicate a scale that is beyond the norm and often convey a sense of grandeur, importance, or awe-inspiring scale.