Its been so long since I ran calculations so I'm rusty but find out what 100 year flood would be for your area. With that small of a watershed the water will only be on the ground for a few hours before it will be downstream of your pond. If the max rain event over a 4 hour period is 4" that will tell you how much water will be on the watershed. If the max flow is say 50 CFS find out what your pipe will handle and then you can figure out how big the spillway needs to go. A 15' wide spillway that is 1' deep will handle a bunch of water.

Go with as big of a pipe as your budget will allow. You wont regret it. Also keep in mind if you will be driving over the spillway often. If the pond is constantly going over the spillway it will stay went and you will rut it up.