not sure about size, but i think there are general rules of thumb re the design of the manifold.. i.e. 4-1 or 4-2-1. (iirc the latter is better for peak power but the 4-1 is better for low down/midrange - don't quote me on it though!)

when i phoned simpsons re an exhaust for my escort they started asking me all kinds of questions that i wouldn't have a clue abotu the answers! then suggested i get in touch with my engine builder and ask them for the relevant info. as said.. massively complicated!

I did have a link to a site that calculated primary lengths and bores based on some basic info though.. will see if i can find it.