With a 16" rim, a 225/60 tire will maintain your factory wheel gap. A 235/55 or 235/60 will be fairly close. The widest you can go is 245. But the largest profile available is a 50. This dimension will
increase your wheel gap by an inch. To avoid the 4x4 look, you'll either need a 17" rim or a suspension modification.
A nice comparison can be found here:
http://www.miata.net/garage/tirecalc.html
Even though you don't have the car you can still play around with how the car will look at either
Tire Rack or
Discount Tire.