^^^^ This.
Dont go by length or age of the snake. Go by weight. Get a postal or food scale and set it to grams. Put a bowl on it, and "tare" the scale (zero it with the bowl on it).
Whatever you feed it should be right at about 10% of the snakes weight. And no larger in girth than the snakes widest point.
I've read that Rats are more nutritious than mice, so consider using rats as your feeders if you are interested in growing to potential.
Pinkie rats run around 8-16 grams... fuzzy rats around 18-25 grams. Weaned/crawler rats run in the 25-40 gram range... and small adult rats run 40-60 grams (these are approximations, in my area)....
My adult is 5 years old, and weighed 900 grams. But she was skittish about adult rats (she got bit once or twice)... so while her meal would be about 90 grams or so... I fed her 3 crawler rats in the 30-40 gram range for each session, and she did just fine with it.