"The boy is tall."
Translation:남자아이는 키가 큽니다.
ㅂ니다/습니다 is a formal verb ending meaning "is/are/am," so 큽니다 means "is big"
As someone pointed out in earlier comments, 키 refers to height. A transliteration of the sentence "남자아이는 키가 큽니다" would be, "The male child's height is big," which is then translated to, "The boy is tall."