I used small ittis on my wee boy from about 2 weeks. They fitted him well and I loved them, they're so cute!
He was 7lbs 9 at birth but gained weight very quickly and was about 9lbs when I started to use the ittis. He has chunky thighs though, and I think that makes all the difference with the fit. He's just outgrowing the smalls now and the medium fit beautifully. There's not much size difference between small and medium.