Snakes have intestines that expand to adapt to the size of the prey item, and they also have really flexible jaws with specialized bone structure and function, your bp should be able to take a rat with no issues. Personally, I think it's better to feed a larger prey item than several smaller preys.
If your snake has issues transitioning from mice to rat, I've heard that rubbing the scent of mice onto the rat can help.
My snake has been fed f/t rat pinkys since hatching, with mice, you tend to reach a point where you run out of the appropriate size to feed to your snake; with rats, finding the correct size prey is easier.