Be sure to utilize the tire calculator below when making your final decision... others can post regarding their experiences with certain types of tires running "slimmer" than others; however, to keep the rolling diameter the same compared to your stock wheels it would be as follows
245/45/18 (stock) ---> 275/35/19 (rear w 19s)
if you want to squeeze a hair bigger in the front and rear on your 19's
255/40/19 ---> 285/35/19
Mess with it and see what you turn up... hope this helps
http://www.miata.net/garage/tirecalc.html