On a turbo motor, it's good to have the headers slightly larger than the ports. Also, large diameter primary tubes aren't that bad contrary to popular belief but you want to keep the length on all exhaust components before the turbo as short as possible.
With my KB headers, I dont use gaskets and they seal great. I use a set of copper gaskets to port from which happen to be a little smaller than my headers. I don't remember where they're from, they've been laying around forever.
To answer your original question, I don't know if they're all the same size but the copper gasket I have seems to be about the same size as some of the ported heads I've seen. I assume the aftermarket copper gaskets are bigger than the stockers but I've never paid attention. Sorry I can't help more.
