How an exhaust sounds means a hell of a lot to most people. Since the performance gains of equal diameter exhausts are virtually identical among all brands, picking an exhaust pretty much comes down to how the muffler looks, how it sounds and what sound you want.
You pick the diameter of the exhaust you need by deciding ahead of time how far you are going to go with your mods and where you want your powerband to be along the rpm range. From there it's just personal preferance on what style of muffler you like the looks of, and if the exhaust tone is to your liking.
I didn't mean to diss on you for picking Spoon, Spoon is high quality stuff. I have Spoon cams, Spoon cam gears, a Spoon shift knob, Spoon magnetic drain plugs and Spoon heat barrier tape on my oil pan. It just that it obvious to me that there was only one reason why you picked Spoon. It may or may be what'll work best for your car with whatever future mods you plan on doing.
If you can tell us:
-How far are you gonna go with your mods (just bolt-on's, or internals also)?
-Where along the rpm range do you want to build power (midrange or upper rpm's)?
We can give you educated advice on what to buy, rather than just spitting out brand names that, when put together, may not work well together towards making max power.