What does 'knee high' mean?

[Holu]

English Language Tests, Intermediate level

ESL/EFL Test #103 "Charlie's Knees", question 6

You see he'd known her once as a small child when he was knee ......... to a grasshopper and now they met as adults.

(a) high
(b) tall
(c) long
(d) broad

English Language Tests, Intermediate level

ESL/EFL Test #103 "Charlie's Knees", answer 6

You see he'd known her once as a small child when he was knee high to a grasshopper and now they met as adults.

Correct answer: (a) high

Your answer was: incorrect
You see he'd known her once as a small child when he was knee long to a grasshopper and now they met as adults.
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I still don't know the meaning of the word "knee". It is not a part of the body. Is it true? Could you explain to me more clearly. I only have two correct anwers.
Thanks a lot
Thuy

[Mister Micawber]

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Yes, a part of the body. The idiom is 'knee-high'. Please use your dictionary.
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Native English teacher at Mister Micawber's