Knee-high to a grasshopper means very small, as a young child is. Knee-high to a grasshopper back then.
It is always about the past and always affectionate. Nobody is literally knee-high to a grasshopper, which is exactly the joke.
1phraseknee-high to a grasshopperVery small, as a young child is.
“She has known him since he was knee-high to a grasshopper.”
knee-high to a grasshopper is a phrase.
Practised in Idioms