Switching from mouse to rat.

I want to start switching my ball python over to rats. What size rat would be equivalent to a medium sized mouse?

You should be between a fuzzy and pup.
How much does your snake weigh?

I dont have a scale so i dont know how much she weighs but i feed her a medium sized mouse on sundays and it puts a tiny almost un noticeable lump in her belly.

Sounds like a weaned rat to me.

I recently made the same switch and from a medium mouse I was getting the smaller fuzzies for rats. There was a 3 gram difference or so.

If a medium mouse is leaving a slight lump, then for a rat it would be large fuzzy or small pup like the others have said. You might want to look into large pups or maybe a small weaned rat but better to start on the small side and work up. Too small of food never killed a snake but too large is another story.