Just keep the widest part of the prey item (girth) as big as the widest part (girth) of your snake or smaller. Up until you get to about a small rat. Then stick with that for its life time.

If you stay on mice it may be advised to feed 2 every feeding. Only in the reason that mice are smaller than rats. Seems everybody shoots for a 40 to 50 gram meal each week as adults.

Hope this helps