The best way to judge prey size is:
1) Eyeball it. Make sure the prey is about the same size as the thickest part of your snake.
2) Use a scale. Size of prey should be 10-15% of your snakes body weight.
I would continue to feed once every 7 days or so. If he has outgrown 1 mouse then you can try 1 appropriately sized rat. If he just refuses rats and will only eat mice, then you will need to feed multiple mice eventually, and in my opinion, this is when a scale should be used so that the combined weight of all of the prey items will be 10-15% of the snakes weight and he will not be over fed...