I breed my own mice and all of my Balls and two cornsnakes get them. The two adult females, an '03 and an '04, both get around 4-5 mice per week. The two '05 females get 2-4 adult mice per week and the 4 '06 babies each get one good-sized adult or two medium adults. The two '06 Corns get a large fuzzy mouse every week.

The late '04 female(Amani) is currently 1650g on empty and 4ft long(I've had her since she was 4 months old and she's always been fed mice). The '03 female(Breth) is almost 1400g, but was malnourished before I got her and only weighed around 850g a few months ago(May-ish when I got her). The '05s currently weigh 905g and 450g respectively(Arith and Hannath). Hannath came from Chris(Cueball) and she was slightly underweight as mice are nowhere to be found in goofy Tyler, TX, unless you feel the need to go to Petsmart where mice are $4.99 a pop. Chris, just to let you know, she's steadily gaining weight and doing great!
The '06 babies(Caylith, Mininth, Orlith and Rineth) are all between 160 and 310g and they do very well on either one fat adult mouse per week or 2 smallish adults. They let me know what they want on the menu by their body actions.

I would stick with the smalls. There is no reason to go with medium rats for a Ball Python unless you want her fasting for 2-3 weeks in a row, which is just frustrating for you.