What is "larger" size of this blouse? What is "the largest" size of this blouse?
No, "What is larger size" is not grammatical. You can say "Is there A larger size?" (comparative) or "What is THE NEXT larger size?" (comparative) or, as the answer sheet suggests, "What is the largest size?" (superlative). Of course, in real life, you might more likely say "Is THIS the largest size you have?" (meaning "Don't you have anything larger?") because you usually don't actually want to know what is the largest size, but rather whether they have one large enough to fit you.