Im currewntly running 6N2 GTI BBR RSII's they are 15" on 195/45/15 tyres. They fit like a glove and I still have enough room to drop my car on coilovers.
It is all depending on your suspension arrangements (standard or lowered)?
if you run say 17" rims you'll need to have a real low profile to keep your speedo true in relation to your actual speed.
basically here is a link have a play around.
http://www.miata.net/garage/tirecalc.html
15" with 45mm profile is ideal as it is same as running your standard 13" with 65mm profile.
with 15" rims if you travel at 60mph you are actually traveling at 59.9mph so a tad slower which in my opinion good as it will keep your licence clean