If I understand this correctly, she refused 1 feeding when she was in shed, then went back to eating, and now has refused 1 feeding when you bought the new rats. Is that correct?

I would not worry AT ALL about a BP that has refused 2 feedings. That is NOTHING to a snake, assuming it is otherwise healthy and acting normal. But I understand owners worry anyway sometimes.

It really is up to you which rat you offer. Some people would prefer to use up the smaller size. Others would go ahead and switch their snake to the bigger size, and from your description, it does sound like the bigger ones are the right size for her.