1) switch to rats all together
2) feed a rat that is 15% of the snakes weight (or the widest part of the snake) until it hits medium rats, and just continue to feed meds the rest of it's life

Mice norm weight about 30 grams so you are feeding about 60 grams to your snake, some stay with small rats (60 grams) other stay with med rats (90-100 grams) for the adult snakes life time either or is fine. If it's growing well 2 mice is fine, if you want it thicker (because you feel it's too skinny and no worries about breeding) add another mouse.