Not saying i am that up to date with the knowledge of gas flow/exahust things but last year my metal work teacher taught me alot about gas flow and sizes of exhaust gas in relation to the size of the engine so ill give it a shot!

What he told me was that the size of the ports, valves and total capacity of the engine has alot to do with the diameter of the primaries/secondaries of the extractors.
Of course tuned length extractors are ideal as then each cylinder or exhaust tract has the same length to travel and same gas scavenging (that the word?)affect and also does not have a difference in pressure.
To know how long each primary should be it all depends on space but also the engines performance. I think he mentioned that professionaly high performance engine builders run the engine with the tube attached and when the pipes turn a different colour that is when they know to chuck a bend or widen the pipe. Dont quote me on this as its been a while since i talked to him and he tended to exagerate a fair bit!

Gas supposedly travels on the outside of the wall so some indentation of press bent extractors aint a big deal but of course mandrel/sand bent extractors are ideal but only when trying to scrape everylast hp out of the particular engine.

When i was desgining my set, we came up with the figures that the primaries should be 32mm and secondaries (as i was going with 4-2-1's) should be round the 39 - 40mm mark for a nice set. This was on a std A15 head and even these are pretty big! he said that this set of extractors should provide enough flow for up to a 150hp engine as the bends came out pretty nicley & flow well.
A set of 4-1's should probably just use the 32mm or 34mm primaries still and say a 2" collector. These will provide enough flow for a nice engine too.

A wall of about 1.2 - 1.6mm is ideal for sand bending (which i had a go at) so im guessing that it should be fine even a bit thick for pressbending as it was really hard to not put dents or fracture the pipe at the tempuratures that was needed to bend the pipe round the mandrel i made. I got as far as making all primaries but not having enough time to finish them off

Thats about all i can think of atm!

Cheers
Dave