Your question is a little vague. Not sure exactly what you are looking to accomplish, so it's hard to answer. Do you want it to look stock, sort of? Are you going to lower it and want them to tuck? Do you want them as wide as possible? In the meantime, here's a link to a site that you can do some comparisons with that I have found helpful:
http://www.miata.net/garage/tirecalc.html