How do i figure out the right collector size.?

i am using a 300zx turbo engine in an off road application and i need to make a custom exhaust for it. how do i determin the correct collecter size, what equations can help me with this?How do i figure out the right collector size.?
man.. you opened a can of worms. first off what kind of headers? then its a 6 right? 3 in to 1 or ? so 2 collectors right? high rpm or bottom end like a jeep?short for high rpm long for more bottom end. but you can do like mine and make them adjustible by sliding pipe over pipe..tune to your liking.make sure you put the ext over the pipe not in it .ok you can buy all the parts over the counter ..lol