Bhai all tires are same, 19/20 ka difference hota hai. Stick to the original or stock size and make sure that the tyres are fresh at least 08 manufactured and go to a reliable dealer who does not stock tyres with tampered manufacturing date.
Check the recommended sizes on the door stickers or users manual.
And also see this tyre calculator, how change in tyre size affects speedo.
http://www.miata.net/garage/tirecalc.html
As far as my research says you can upsize tires upto 3% difference