Header size and Brand

With "Pipemax" its pretty easy to ballpark the tubing sizes - especially for a 4 into 1.
I can tell you from experimenting with it that rpm of interest is a big factor, as is cam and volumetric efficiency. The more efficient the engine, the smaller tubing needed. The higher rpm, the larger tubing needed.

Or, you can go by Calvin Elston's guidelines:
What tube size off the head?

If you go with a size that matches the port, or is a little smaller, then two points that may not be in that particular post.
a) Make sure the top and then the sides, do not hang down into the exhaust port.
b) Step up the tubing size 8" to 14" out.
If the primaries off the heads are already larger than needed, having a step up may not gain much if anything. It might even slow the low rpm exhaust velocity down even more.