Feed your ball python once a week. A lot of people mistake roaming around as a hunger sign but it's not. They need to have fun too and sitting inside a hide 24/7 would make anyone crazy. If fed once a week, your ball python wont starve and will grow at a healthy and safe rate.
If your ball python is about the size of a dollar, probably a pup rat. The best thing you can do is get a string and use it to measure the widest part of your snake's girth and take that with you to the pet store and ask for something that measures that width.
You have a higher chance of getting bit by removing your snake from in enclosure into a separate feeding tub. People follow the myth that feeding the snake outside of his home is better to reduce the chance of getting bit but that is totally the opposite. Also, feeding outside the enclosure is very stressful and sooner or later your ball python may stop eating because of the stress moving him includes.