If you can weigh the snake and the mouse, usually a good rule of thumb is that the snake can eat 10-15% of its weight. If you don't have access to a scale, you can probably guess and feed it prey no larger than the largest part of the snake's girth -- I would think an 8 month old BP should be able to take a small adult mouse without any difficulty.

Also -- do not use your hand to dangle the mouse! If your hand is warmer than the mouse, it could accidentaly strike your hand instead. I use hemostats which you can pick up from most of the online herp companies. Otherwise, a set of kitchen tongs would work in a pinch -- as long as they don't have sharp edges!!

Later,

George