Go here or have your engine builder go here:
HeaderDesign.com 
Register at the site and use their calculators. You'll get accurate information for your particular engine's characteristics (including things like "Exhaust Port Length in Cylinder Head", "Performance Factor (how will you use this engine)", etc. You won't be sorry (and you'll probably find out that VintageMusclecar was right on the smaller tubes). Then you can find the right header manufacturer for you based on price, fit, etc.
The big tubes always look tougher but aren't necessarily the right performance answer.