knee-high to a grasshopper
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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.