The max size I would put on any early 240/260/280 Z car would be 17 inches. Why? First off, if you took a physics class in high school you learned that the greater the diameter of the cylinder, the longer the revolution is to complete, thus effecting acceleration. Now, on the street, you probably can't tell the difference between 1 inch, but once you start up-size 2, 3 or even 4 inches, you can tell. The benifit is a higher top speed, but how many times on the street are you going to hit 145mph? I would rather accelerate quikly.
The other factory limiting wheel size is the fenders and suspension on the Z. It already has a low ride height to start with, but when you throw on some 18 inch wheels and go over a bump at 60 mph you are nearly garrantted to rub your fenders with the rear tires. Sometimes, if you're lucky, it will just be cosmetic damage. But, most of the time, it can rip your fender apart, cut your tire down, and cause you to lose control of your Z.
The Z is rear wheel drive, so it makes sense to have wider rims in the rear. I like 16 x 7 inch up front and 16 x 8 inch in the rear with fender flares to cover the tires that protrude outside the stock fenders. The front tires size could be a 215/45-16 or 225/45-16. The rears could be a healthy 245/45-16 or even 255/45-16's. A nice look could be front 16 x 7 inch wheels with 215/45Z-16 tires and the rear rolling on 17 x 8 inch wheels with 245/45Z-17 tires. A very nice look.
Many "tuner" and aftermarket companies make wheels compadible with the Z. They use a 4-bolt hub which is still used with cars today. Some of my favorites are the Konig Imagine and Hurry, both of which are 17 x 7 inch wheels. Other wheels that look nice are Panasport Formula's that come in a varity of finishes including gunmetal, graphite, silver and gold. Most Panasport's are 16 x 7 inch rims.
Now for tires. The original factory tires were measly 175/65-14's! Honda Civic's now come with wider tires! It's time to upgrade to some larger meats. Here are some great tires choices for the Z:
-Bridgestone Potenza S0-3 Pole Position
-Michelin Pilot Sport
-BFGoodrich G-Force T/A KDW 2
-Kuhmo ESCTA Supra 712
-Yokohama AVS ES100
-Toyo RA-1 Proxies
All these tires are available in multiple sizes and are bound to fit the 15/16/17 inch wheel you've choosen. Some people like to go the old-school route and stay with 14 inch wheels but with performance tires, but those tires are getting harder and harder to find at a decent price. I wouldn't go lower than 15 inches, but that's just me.