"Essentially a long tube length and small tube diameter will assist in low rpm torque (this goes back to the scavenging effect). Large tube diameters and short tube lengths promote upper rpm power (because the scavenging effect is lost to lower rpms but gained at upper since the exhaust flows so freely).
This said, header design (tube size, style, length) is the most important factor when it comes to selecting one for your application, not 'who made it' or 'how good it looks'. Selecting the wrong header leads to an engine that can be a real DOG (because the engine is designed to build power at "X"rpm and the header is designed to promote power at "Y"rpm).
Another 'myth' is that a turbo needs HUGE tubes to operate. Again, this is a MYTH. Too small of tubes makes the exhaust back up into the engine (meaning that you are contaminating the intake charge) and too large of tubes means that the turbo is not spinning optimally. Everything AFTER the turbo can be 20" diameter for all it matters as long as it flows well."

After reading this, I've got a question (most likely a dumb one). If one were considering installing a turbo, then would it be effective to install headers with long tube length and small diameter to increase low end torgue to offset the "lag" turbo's create while they spool? If this is the case, which headers would one choose?