Bigger is best - unarguably so on a turbocharged car. Post turbine, these boosted engines need to breath as much as possible and - therefore - the largest diameter tube will be able to carry the gasses with the minimum of restriction. Three inch is presently the largest commonly available diameter you can get - though 3½ and 4 inch are now starting to make their debut. There is some argument that a smaller pipe diameter (and therefore more back-pressure) is required for optimal performance on a naturally aspirated engine. This theory, however, is unproven where the fuel and ignition has been optimised to suit the pipe, and where the large pipe is used after the tuned length part of the system.