I would invest in an inexpensive scale 10-15% of body weight is the best way to go with feeding I try to stay away from mice if possible because Ball Pythons cna be very picky about what they eat and when your snake gets full grown you will have to feed 2-3 mice in a meal. I would think at that size a Rat fuzzy or Pup would be OK but again without a weight it is hard to tell

Here is a general size comparison chart for you hope this was helpful

MICE

Sm. Pink 1-2 grams
Lg. Pink 2-4 grams
Peach Fuzz 4-6 grams
Fuzzies 5-8 grams
Hoppers 8-12 grams
Sm. Mice 11-16 grams
Med. Mice 15-24 grams
Lg. Mice 24-28 grams
XLg. Mice 30 grams+

Rats:
Rat Sm. Pink 3-8 grams
Rat Lg. Pink 5-12 grams
Rat Fuzzies 10-20 grams
Rat Pup 20-30 grams
Rat Wean 30-45 grams
Rat Sm. 40-80 grams
Rat Med. 80-150 grams
Rat Lg. 140-200 grams
Rat XLg. 190-250 grams