Fig.: to be very short and, plausibly, very young

Do I know him? I have known him since he wasn't even knee-high to a grasshopper

Se lo conosco? Lo conosco da quando era alto come un soldo di cacio

He would go to any lenghts to become a professional basketball player, but he's knee-high to a grasshopper!

Farebbe carte false per diventare un giocatore professionista di pallacanestro, ma è un tappo!

I remember that I was a half-pint and I was already going hunting with my grandpa, even if – luckily – I couldn't even hold the shotgun

Ricordo che ero alto come un soldo di cacio e già andavo a caccia con mio nonno, anche se – fortunatamente – non riuscivo nemmeno a imbracciare il fucile

This US expression replaced in 1850 the earlier Knee-high to a toad, to a mosquito, to a bumble-bee, to a duck and to a splinter