Yes, your bp will do a little breakdance after it eats it.

Usually you gauge your prey size by matching it to the size of the thickest part of your snake. It doesn't necessarily have to be a bit bigger. As long as the rodent you get it the same size as your snakes thickest part, it's perfect. A litle bigger just means you'll see a noticable lump and it's a bit more filling I guess.