205/55R16 was stock on the 98. 215/50R16 was stock on the 99 and 00. Either size will work fine. The 215 is slightly wider and slightly smaller in diameter.
Actually, there is another size that works well too. It is 225/50R16. That's the size I have.
It seems that the 215/50R16 is a slightly unusual size and you have much more choice with either of the other two.
Do a google search for "miata tire calculator" and play around with it a little and you will get a better idea about fit.
