Follow this chart and you'll be fine.
There are two general rules of thumb. First, the prey item should be approx the same size around as the snake at its widest point. Second, the prey item should be 10-15% of the snake's body weight. The guidelines change as the snake reaches adult size, but this will get you started.
I wouldn't put too much into what that store is telling you. Whoever told you to feed a BP pinkies between meals is either clueless or trying to take advantage of you to increase sales.
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