Yup, you are being paranoid. He or she will poop when he's ready. Mine rarely go and it drives me batty. Are your temps right etc? Some say proper belly heat helps them digest, some disagree.
As for the mouse... It would be rare for a mouse to be too big as long as the snake is well beyond the hatchling stage. Best way to gauge prey size is the snakes weight. By grams, the prey should be about 15% of the snakes weight.
Thank you. (: I own many Cornsnakes and I fell in love with a BP at a Reptile expo. Anyways I wont be on later. Have to drive home from AZ from a herping trip. (: