300g for a female corn seems REALLY small to me. I would go more for 450g-500g.
As for Pueblans, they are a pretty small snake. 300g might be about right for a first time breeding female. The Milksnake book by Robert Applegate has tons of breeding records with the female's weight being recorded. It's also a handy book to have around for other reasons.... best Lampro book I've read... IMO