Weigh her tonight and go from there. It's much more accurate to recommend what she should be eating by weight than anything else.
Most people use one of these suggestions: feed a meal that 10-15% of your snake's body weight or feed prey thats is the size of thickest part of your snake.
Here's a chart of approximately how much the mice and rats weigh. If you want to be more accurate, you can always weight the feeder mouse or rat:
Regarding the switch from mice to rats, in my experience most of them switch over pretty easily. You can always scent a rat with some mouse bedding to make an easier transition.
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