When you start seeing marks then you move up a size my baby is size 5 and my toddler is a size 6 always did it that way
With my toddler, I always sized up when the diapers started to leak regularly assuming no leaks otherwise.
We went straight into Aldi size 2 as they're from 6lbs and found the size 1 always marked his legs even when he was first born x
Sometimes on the main package it will have guidelines as to when to move up a size e.g. the straps being so far apart when you do it up. But generally if it's leaving marks or leaking repeatedly then move up a size
I'm not sure, but I ran out of size 1 and ended up using size 2 nappies I had in the house. We had no accidents with the size 2s x