The first, with the modification “an” is the correct one. When the noun ‘size’ is used (even when modified) it takes an article—in this case, “an”—because it is preceded by “exaggerated”. The word “exaggerating” is a verb, so is not grammatical in this sentence.
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yzh1978Why is "She has an exaggerating breast size"It is grammatically correct, but the meaning is nonsense.isnot right?