Show or go?
Either way you look at it a 17 inch tire on a stock height Miata will look pretty funny. Big rim, little sidewall, lots of space in fenderwells... but its your choice, you will be the one that has to look at that gap everyday not me... but then again how long do girlfriends stick around?
If you are like the way I was when I was single you'd be free of her in a couple months and be able to start spending money on car parts and toys instead of dinner, movies, and girls clothing.
Good luck (thumb) with the car... and with finding a more excepting girl, j/k