Get a rat equal to.the thickest part of the snake, or something near it if you want to go a bit smaller. It's not really efficient to say medium or large because your definition vs mine vs where you purchase them at will vary.
Getting pray close to the size of your snakes girth is a safe rule to go by. You can try that over the course of a couple of weeks and make any adjustments (in pray size) if the need arises.
I disagree with the above posters. Once a snake gets that big i would NOT go by the girth rule. A 125g rat once a week is sufficient. A 150gish rat is the biggest live rat i'd ever feed and i think thats considered a "small" medium or a "big" small by most people. If you go by the girth rule you could be feeding her like a 300g rat which is too much food and dangerous. All this is just my opinion, but i'd be very careful if you choose to disregard it and feed larger live prey.
I use my 3,000 gram female mentioned earlier as a garbage disposal. She has eaten jumbo rats, 300 grams plus, several times. She never regurges and never has trouble digest. I wouldn't do it every week, but it's never hurt her. Balls aren't true red tail boas or emerald trees that are prone to regurge. Do you really think a wild Ball would pass up a meal that size? Not trying to argue, this is just my experience.