Keep in mind that two bigger ones are way superior to 4 smaller ones, pipe volume quadruples every time the size doubles, inotherwords, it will take four 8" pipes to carry the amount of one 16" tube.
Its basic math from there, the 18" culvert will carry almost 1 1/2 times the amount of water the 15" one will, its kinda crazy how that works.

That being said, like Esshup said, until you get an engineer to do the math you are basically making a stab at it, so you want to be sure you oversize it, and I have had engineers be wrong too. the county here will calculate what size culvert you need to install when you obtain your permit. I have had a 4' dia CMP (per their recommendations) wash out and get washed a hundred ft down stream after an extremely heavy rain event immediately after installation.

All the really good ideas I've ever had came to me while I was milking a cow.