1
general::
Quite young, as in I haven't seen him since I was knee-high to a grasshopper. 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.
American Heritage Idioms
2
general::
Phrase(s): knee-high to a grasshopper
Go to *knee-high to a jackrabbit and *knee-high to a grasshopper
McGrawhill's American Idioms And Phrasal Verbs