If you are staying N/A with your car you really want to stay under 2.5" Diameter. Make sure that if you are going with quality that the pipes are MANDREL bent, not CRUSH bent. The difference in air flow between the two is incredible.
If you are planning on going forced induction or a large shot of n02, then you might want to think about 2.75 mandrel bends.
If you want 500+ HP then 3.0 is the exhaust you want to go with, this might be a problem however as we are limited by the space inbetween the trunk and the trailing control arm...
When you say custom exhaust... Do you mean header back? if yes then understand that unless you spend a crazy amount of money on a header you are going to have to modify the downpipe it comes with, you also have to think about silencers or resonators.
I have a 4-1 2.5 inch header with 2.5 inch mandrels alll the way back with no cats, I kept the resonator though, just to quite it down.
You also have the option of a glasspack, which will sacrifice 1 - 2 ponies, but will quite her down a lot....
Also depends on what type of muffler you are going to install what is the inlet size? will you have to upscale the pipe to the muffler or downscale it?
there are A LOT of options for exhaust for our car, well any car really, and it is going to be the most noticeable thing about your car. Think about local laws and talk to people here with different setups before you decide.
Sorry for being so long, but this is important for a new person to understand