CyberGhost, you have probably heard it somewere before.
When it comes to ball pythons, it is best to go with food no larger then their head for the first few months of their lives, until they become an established feeder. Anyone who has raised ball pythons can tell you that they can be finiky eaters.
Once they are a few months old then you can graduate the food size to match their girth safely with less chance of regurg..
This is just a rule of thumb that I and many other ball python owners go by. Over 20 years following it with no probs
And always feed dead pray whenever possible, or at least stunded as to protect your snake from bites.