Its not so much the fact that you have bigger wheels that you have to worry about, its the overall diameter of the wheels as they compare to the spare. Check out this site:
http://www.miata.net/garage/tirecalc.html
If you enter your stock tire size and compare with 215 45 17 (common 17 tire size), you'll see there is a very small variation in the overall diameter of the wheels as compared to stock.
I think that is acceptable for short distance driving on your spare. However, if you are using different size tires than that, you may have to run with a full size spare for emergencies.
Of course, if you do decide to go with a full size spare, all you have to worry about is putting the same size tire on the same size wheel as what you are running. It could be *any* 17 wheel, unless you're worried about them matching in the event you have to use the spare. Try to pickup a single wheel from someone who has wrecked their car and has an odd number of good wheels left. Try to find a used tire in the same size you're using to save some money, and consider that chances are that if you get a flat, it will be on a somewhat worn tire. Ideally, you want the tread wear on all your wheels to be as equal as possible.