The phrase "a whale of a" is often used to describe something as big, impressive, or noteworthy. However, there are several antonyms that could be used to describe the opposite of this phrase. For example, one could use "a mouse of a" to describe something small or insignificant. Alternatively, "a drop in the bucket of a" could be used to describe something that is not significant or substantial. Another antonym is "a mere speck of a," which could be used to describe something that is barely noticeable or insignificant in size. These antonyms provide a contrasting perspective to the original phrase, highlighting a different aspect of the object under discussion.