best way to tell ( i think) when they are ready to bump up in prey size is you feed it the usual 1 mouse or rat a week and it is hungry roaming around half way threw the week. then it is very clear your snake needs bigger food. i try to keep it where mine start roaming around around mid-day day 5 to 6 that way they get some exercise at least instead of just eat and lay there all the time. i also skip a meal once every 2 months (but i also feed couple different prey items mice for babies then large rat pups asf rats and baby rabbits when snake can handle them and theres any around) my blood wont touch a baby rabbit dead or alive but throw a mouse or any kind of rat in there and she hammers them live f/t or pre-killed seconds before she gets it