personally, ive gone with what i feel mine can handle. Look at the widest part of your snakes body and judge mentally where you think they will be in 2 months, 4months and one year. they grow fast! so dont underestimate the size change..

My current baby pastel started with hoppers and she devoured that little dude quick, so i threw in a weaned mouse as well. right now she is around 110g (has gone from 69g to 110g in a month) and ive bumped her up to one small mouse every 5 days. she will likely be up to regular mice very soon because she downs the small mice so darn fast.

I think most will tell ya, get your ball on rats as quickly as possible. Mice are like candy My one adult boy took forever to totally switch over to rats. He was pumping down 4 mice at a time before i got his butt switched over to rats completely. Look at a size comparison from mice to rats. At 12g for a mouse - id start at rat pups but youll likely be up to weaned quickly.