First, look at your current tire size. It should be written on the sidewall of the tire in the following format:
eg: 205/40R17
In that example, the 205 means the width of the tire, 40 means the profile, R means the tire is a radial (all tires these days are radials) and the 17 is the size of the wheel (17")
In order to not throw off your speedometer reading, the radius of the new tire will have to be the same as your current one.
So, it seems the wheel you want is 20" in size, and 8.5 wide. This means that the tire size you would want for it would probably be:
245/xxR20 (245 is the tire width I'd imagine you would want, and the 20 means a 20" wheel)
To find out what the blank part (the profile) should be, go to
http://www.miata.net/garage/tirecalc.html
On the left, enter in the stock tire size you read off your tire. Then use the right side to find out what the correct new tyre size should be, keeping in mind that you want the speedometer reading to be as close as possible to how it was before.
=]
www.miata.net