키 height 크다 to be big -시- prefinal ending that you can use in order to show your respect to the subject 어 ending for the vowel excluding [ㅏ], [ㅗ], you can use it to your close friends you can talk about anything. 키가 크+시+어 = 키가 크셔

키가크 셔.(X) → 키가 크셔.(O) 키가 크셔 is the exactly same with 키가 크시다. If someone who is older than you, you can say "그/그녀는 키가 크시다." if someone who is younger than you, you can say "그/그녀는 키가 크다" You got it?!!! lol~~