My normal was eating f/t large mice from Petsmart, but they seemed small and I really wanted to go to rats, but them seemed way to big. With that said, I have heard two things when it comes to feeding balls. One is to feed 10%-15% of there body weight or feed them prey according to the width of the largest size of their body.
I ended up finding a local reptile store were I picked up a small frozen rat. I didn't weigh the rat or the snake for the first feeding (just based the prey on width of snake) but today was the second time of feeding my normal ball a small frozen rat. The ball weighed in at 236g, and the rat defrosted weighed in at 76g, he took it like a champ, much quick than the last rat. Since he is taking bigger prey over the 10%-15% rule, I have moved him to a 7 day feeding schedule verse every 4 days to allow extra time to digest. The rat of course did weigh more than 15% of my ball but, girth wise, was not much bigger than my snake, he seemed to handle it just fine.