We've had a 13" nardi, a 14" leather and a 15" nardi in the spyder (roughly sized, they were actually millimeters). The 15 was way too big, the 13" is perfect for Steve but I hate it. The 14 was leather and we wanted the wood look. So the car has a 13" on it.
One thing to consider. As you move to a smaller wheel, you "quicken" the steering response. Bigger wheels, make the steering slower. It's a matter of movement. With a big wheel, you might be moving 3" to get 5 degrees move movement, but the smaller one may only be 2" so it feels quicker. A big wheel gives more leverage (moot point on spyder) and may make the vehicle feel a little more settled at speed.
The 13" blocks part of the guages even for me in the spyder.
angela