Paul, the prey item should be big enough to leave a noticeable lump in Plissken's belly but small enough that he can swallow it. At 21" an adult mouse ought to fit the bill. The feeding response doesn't typically shut down immediately after they eat, though, and mine will all cruise after eating but generally refuse a second prey item. Glad you got him to eat!

Aaron, we might be able to help, but a little more info would be useful. How old is your bp, how long have you had him, how often does he normally eat, what size prey does he normally eat, etc.?