Not a simple calculation as figuring out how much water your creek runs, its a matter of how much it runs on that biggest rain event day, and be able to handle it without washing out.
Its a matter of calculating how many acres drain into it and a bunch of other variables, such as what vegetation, if any, is on that acreage and how steep is it, which will relegate how fast its gonna all get to your overflow, and how much freeboard you have if your overflow system will not handle all of it immediately, hopefully you have an emergency spillway for such an event.
Not a bad idea to consult an engineer to put some numbers together for you, could save you money and or headaches in the future.
Good Luck! sounds like a fun project that everyone is looking forward to.
