You would use the phrase "¿Qué talla de falda usas?" for a few reasons using context clues. You don't even have to know what it means, but you do need to know that Ana asking a question to someone (Isabel) that she uses the informal "tú" with. You also need to know the conjugation of the verb "usar" in this sentence. Asking a question to a friend? Use the "tú" form "usas." Talking about yourself? Use the "yo" form "uso."