what is the biggest wheel and tire I can put on my car without major modifications?

You can use this site for plus sizing. It shows the effect on the speedometer and odometer. --- http://www.miata.net/garage/tirecalc.html

go bigger diameter wheels with low profile tires to HELP cancel out the difference. I put an inch bigger on my car for a while(15" to 16") with low profile tires, and the only issues i had were related to the width.