I personally use 1/1/2 pipe if your moving turbo away from the head ie over gearbox etc to keep exhaust gas speed up and 1/ 1/2 tube is good for 500 bhp .
If your running turbo on short runners ie next to the cylinder head like cosworth then I would move up to 1/ 3/4 , this would be good for 600bhp .
The main aspect is really trying to get all the runners the same length which is a lot easier than it sounds with nice radius curves and try to get as least welded joints as possible as for every weld you've got one more area where a crack can occur.
Material is the next major factor , I use 304 and never had any problems on turbo car running 500 +bhp but if you can afford it use 316 stainless

Hope this helps

Cheers Paul