Hello James, Not being negative, but I simply dont know the dimensional answer to your question, but, despite many ideas about sizing, usually the primary pipe size is related to exhaust valve and port size.
Just think of it as trying to provide the smoothest possible exit route for the hot gasses with either, nil, or only smooth changes of outlet/primary pipe cross section. Indeed, the sort of arrangement achieved in most if not all cast iron manifolds.
Choosing pipe size then, may be just one of a number of issues, but it is also determined by collector arrangement, available materials and space around the engine and beyond. There is also the reasonable conclusion that as the pipes merge then they also need to grow in diameter to cope with the accumulating volumes of exhaust gas at each junction.
But what about length? Primary pipe length is dependent on wave tuning. Typically, longer pipes tune for lower rpm power and shorter pipes favour higher rpm power. So, use determines best length.
And what of the other considerations. Types of silencer will affect performance as will the thickness and type of all materials used (headers/pipes/silencers). For best performance, the thicker the better, because this keeps the exhaust gasses hotter, so less dense, requiring less energy to move them along. Or, inversely, colder gasses are denser, heavier and require more effort to keep them moving. (anyone voting to bring back cast iron manifolds at the election?
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Silencer type? Absorbsion (simplest), restriction (worst/cheapest) or relection (sophisticated), pretty much self explanitory, but each having a different effect on the way gasses move thro the system. Difficult choices when one knows the differences.
On the other hand do we just choose silencers by whats on the garage floor or what one can get from auto jumbles or Ebay.
If the above choices seem fatuous, they definitely are not, because they do make a difference to efficiency, which sorta makes worrying only about primary pipe diameter whilst ignoring everything else, slightly incidental.
Especially as my first observation is probably the best bet for primary pipe size.
You can read much, much more about this whole subject in my PDF on exhaust systems available from this link:
http://www.vintagemodelairplane.com/pag ... ust01.html