I feed mice so I'm not as familiar with rat sizes, but I believe a fuzzy rat is in the right size range for a baby BP.

A good rule of thumb for BPs is that the widest part of the rodent's body should be about as wide as the widest part of the snake's body.

If you have a gram scale, you can also go by weight, feeding about 10-15% of the snake's weight at each feeding.