I built a creek bed 2 years ago (when I got the tractor), it's seasonal only, but needed to control the water to the pond and property as the valey emptied into the back 40 and the water just went everywhere.
I used a 16"colvert but found out last winter durring a significant "Wet" period of time that it wasn't enough for "All" instances. I will be installing an addtional 16" colvert next to the orrigional to manage the "Occasional" flow demands. The main road crossing is 16" and that's why I went with that size orrigionally... but I should have known better as water has backed up from that for 2 decades..... (what was I thinking??)
I'd say, what ever you think the fllow is on "Average" plan for double that at least. That way you have 1/2 a chance of avoiding scouring out of your crossing and having to re-do it.... like me.
Cheers