It's sad when a direct google search works better than an onsite search.
Here's a link to info on offset
zerothread/158337
I don't think there is an "optimal" rim size. This is what you have to take into consideration:
The stock rim for a 4th gen is 15 inches. There is an article online that says that the 4th gens are the fastest amongst the 3rd through 5th gens.
The basis for this was the weight of the car and the rim size/weight.
So a 15inch rim will give you faster acceleration, slightly better gas mileage and a more comfortable ride.
17 inch rims will decrease accleration and possibly top speed. Your gas mileage can suffer and the ride will be a little more bumpy. However, you will be able to have shorter, wider tires which equates to much better handling and harder sidewalls.
In order to use a 17 inch rim you will need a wider and shorter tire. The recommended size is a 235/45/R17 because it's within 2% of the stock rim/tire combo diameter. However, if you go with a 225/45/R17 you can save yourself money and buy used tires from 350Z and G35 owners who changed tire and or rims. The downside is that your spedometer will be off by about 1 mph for ever 20 mph.
Going over a 17 gets into problems. You start rubbing the wheel well plastic more and tires start getting more expensive.
Hope that helps.