General rule of thumb is 10-15 percent of your balls weight; some also judge what they are going to feed by the largest part of the balls body. In terms of what to want to feed, wither it be live or F/T, I think it is personal preference. Some prefer F/T as it is potentially safer for the balls as the prey gets bigger. Bigger prey equals bigger teeth; you don't want a decent size rat trying to naw on your ball if the first strike is not optimal. If you do feed live, I would just suggest having something around to shove in the rat’s mouth if it goes to bite your snake, e.g. pencil. F/T definitely has its convenient factor too, as you can stock up on food without having to make a trip to your local breeder every week.