With my oldest, we did mostly sized. The BGOS came out (1.0) when he was 4 months old, and they fit then. I had a stash of the 3.0 for a bit during his toddlerhood, but he out grew them at 2.5 in the rise, even though he was slim, and we had to use large diapers until he PL at 3.
Dd stayed in newborn diapers until 3 months, but I didn't like the fit of one size until she was 4 months. She is only 14 months now and on the biggest snap settings of several brands of one size diapers. She has been walking full time for 6 months so I don't expect her to slim down much longer. One size certainly won't last her to potty learning.
Mama to Henry (01.07), Catherine (09.11), Audrey (04.14), Clare (09.15) and our newest addition, coming July 2017!