Wiktionary
knee high to a grasshopper
a. (context idiomatic English) short; especially relating to when the subject was a small child
Usage examples of "knee high to a grasshopper".
A brisk little stewardess, about knee high to a grasshopper, checked my name off a list and offered me an injection against dropsickness.
Tommy laughs, 'Jeez, Mole, I must have told you that one when you were knee high to a grasshopper.
Hiram had been working in the store since he weren't but knee high to a grasshopper, and I guess I'd gotten accustomed to him.