être haut comme trois pommes

to be very small

An endearing way to describe a young child's tiny stature.

Je me souviens quand tu étais haut comme trois pommes et maintenant tu es plus grand que moi !

I remember when you were knee-high to a grasshopper and now you're taller than me!

Nobody's sure where it comes from. It showed up in literature around 1910, and Peyo later used it to describe the Smurfs, which probably helped it stick.