You feed a prey item the same width as the largest part of the ball python. Ball pythons can start on rat pups/mice hoppers but will soon move to adult mice quickly. They are also pretty notorious for being picky eaters so just stay with mice or if you would like, switch to rat pups now. Switching sizes of prey items is not advisable. It technically won't lead to refusals if it's just mice, but only feeding every 5-7 days, wouldn't you want something in its belly that's not going to be digested on the second day and you are left with a hungry bp and can't feed for another 3 or 4 days?!
I should add that, I'm regretting starting my one female on mice, which is what she was on when I got her but should have tried to convert to rats. If you stick with mice, you will end up feeding multiples because unless you can acquire large/retired breeder mice, which is the largest sized mouse available and hard to come by since it takes a few years for them to get to that size, you could be feeding up to 3 mice a feeding, which is ok, but costly in the end. She also started refusing mice and now only eats ASF's.
Read this page: http://www.canadianmouseman.com/eng/products.htm
That shows the different prey sizes available for mice.
Cheers