I didn’t know what size the socks would be when I started making them with US 3 and DK yarn – they looked wayyy too small. I changed to size 7 DPNs and ww yarn and they have a 5.25″ sole length.
So that would be for approximately an 18 month old toddler.
This is the closest chart I could find from Petite Feve:
Also, awesome hat sizing chart from Hello Lovely.