Certainly the bigger the better, but if you want to put mufflers on, then you're pretty much limited to 3". Also 3" tubing is the largest readily available. This makes it the most cost-effective size. Check the classifieds for available systems.
i already haev items lined up for purchase thanks to help from chimmike, ive read alot of u guys' posts about turbo and read alot about shadoff's car (thank you by the way) and just learning about turbo. im gonna go with the 3 inch on this one. thanks guys.