You can use this calculator to help you with the size. The stock size is 225-50-16.
http://www.miata.net/garage/tirecalc.html
But with those wide wheels I would say you could run a 235 in the front and 255 or wider in the rear. The only problem is to keep the stock overall diameter you are going to have to run a pretty low profile tire. Just curious but what is the weight on those? And if you are going to lower the car also be prepared to feel the road, like ever bump. I've had 18's with eibach springs and hated the ride, so I will be going back to 17's.