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]

.
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

[Watie]

be knee-high to a grasshopper
informal humorous
to be very small or young

(Definition of be knee-high to a grasshopper from the Cambridge Advanced Learner's Dictionary)

[Puppet]

Hi,

From where this terminology came from?

We used to go through Oxford dictionary. But it's not mentioned there.

[Watie]

Please, check this out:
be knee-high to a grasshopper

[Puppet]

Hi,

Thank you, Watie.

I was asking: from where the idea of 'knee high to grasshopper' came from?

[Mister Micawber]

A grasshopper is a common insect, and insects are very small. This hyperbolic expression, dating from about 1850 and alluding to someone's youth, replaced the earlier 'knee-high to a mosquito' or 'bumblebee' or 'splinter'-- all very small things as well.
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Native English teacher at Mister Micawber's

[Puppet]

Thank you, Mister