Hi,
The general rule is 10-15% of bodyweight of the snake. Also aim for something about the same width as the widest part of the snake.
Generally BP's start out on hoppers and move quite quickly to small adult mice.
But I wouldn't worry about the lack of poop so far - if she was on fuzzy's for most meals and has only had 6 or 7 meals in total then she may simply have nothing to poop.
I would keep her on one meal every 5 - 7 days using the guidelines above.
dr del