exhaust mods

I got 1996 civic cxi (1.6L SOHC non vtec), i wanna get a full stainless steel exhaust system + high flow cat.
What size of the pipe should i get, 2.25" or 2.5"? and do i need to do anything to my O2 sensor? or alter the computer chip configuration of my car after done the full exhaust mods?

thx guys

Well, your cat is probably right of your motors head, and most manufacturers make exhaust for models that have it behind the motor at the end of the manifold, so you'll probablt come up about 1.5 feet short. And 2.25 is optimal for most N/A applications.