"Their granddaughter is tall."

Translation: 그들의 손녀는 키가 커.

[Dilini2]

키가 크다 - tall
키가 작다 - short

[Ronnaroo]

What does 커 mean

[wintertriangles]

It means "big" but "키" is size so it's saying the size is big which they use for saying people are tall.

Oh right, 커 is the most casual form of 크다. It can be 큽니다, 큰다, 커요, or just 커.