Where I get my Rats from the sizes are as follows:
Pups (20 – 29g) 14 – 19 days
Weanlings (30 – 49g) 20 – 25 days
I would probably be feeding pups depending on your guys feeding response and girth.
Lot's of people go by the logic of 10-15% of your snakes bodyweight for feeding or a rat that is roughly the size of the thickest part of your snake.