The best exhaust diameter for your car really depends on how many modifications have already been done and also the desired outcome and/or application.
I won't bother delving into which headers are best to get for your car because there is a lot of argument as to which is 'best' header. Search the forums for more info on headers.
Here is a rough guide as to exhaust piping diameters for common Honda engine capacities.
NOTE: The following is only a rough guide and is in no way appicable to all cars
Non-VTEC
1600cc - 2 Inch
1800cc - 2 or 2.25 Inch
2200cc - 2.25 Inch
DOHC VTEC
1600cc - 2.25 Inch
1800cc - 2.25 or 2.5 Inch
2200cc - 2.25 or 2.5 Inch
*All values are internal diameter.
The second set of values given are if you are after a strong top end however it should be said that you will be sacrificing low RPM torque as a result. Most exhaust shops will advise you of the lower value to give you a good spread or power throughout the rev range, but if you are after good power for race situations, I would advise going for the second set of values.
The key is to have the same diameter throughout the system from the collector all the way to the muffler for optimal performance. There is a common saying "A chain is only as good as its weakest link" and the same applies here in terms of keeping a constant diameter piping all the way through.
