Thank you for your question. I assume you are asking about food as it pertains to your ball python. According to David and Tracy Barker in their book, Ball Pythons - The History, Natural History, Care and Breeding, the goal, when feeding, is to feed a single prey item large enough to create a barely discernible lump in the snake's body that lasts for two to four days.
So the question is how large of a mouse or rat do I need to feed to create this specific condition? The answer is it differs for each snake, and you find out what is best for your snake through trial and error. Always start out with smaller prey and work your way up. Don't feed a prey item you suspect will be too large and work your way down. If the prey item you choose doesn't create a barely discernible lump or the lump doesn't last for two to four days, then, on the next feeding day, provide a slightly larger prey item to see what results you achieve. Of course, as your snake grows, you'll have to adjust the size of the prey.
The book I mentioned above is an excellent reference for anyone interested in ball pythons. It is pretty expensive, but you may be able to get it through your local library's interlibrary loan program. Here's the full bibliography:
Barker, David G. and Tracy M. Barker. (2006). Ball Pythons - The History, Natural History, Care and Breeding. Boerne: VPI Library.
I hope this answers your question, and thank you for contacting Ask an Expert.