Can you give us a little information on his home? What are your temps/humidity?

The best way to tell how much to feed them, is to weigh them. The pet store should have a scale that you can weigh them on. You can also by one. I bought one on ebay for $15 and that included shipping.

They should be fed 10% of their body weight. If she weights 200 g's then feed her a 20g mouse. Most balls can eat hoppers as hatchlings. The prey should also be a little smaller than your balls largest circumfrance.

You can try a small mouse. Just watch her carefully. If she doesn't eat it take it out.