customs;1869840 said:
http://www.supramania.com/forums/showthread.php?56843-Wheel-Fitment-amp-Question-Thread
Read through that thread. There is life times worth of information, more then you will need.
This is the second time (that I've seen) that the OP has been given that link.
To sum this up in a quick nutshell:
You won't go too low on drop springs alone. The MKIII's you see that appear to be "laying frame" are all on coilovers with extensive suspension and fender work. Some guys get away with 17's and not need to roll fenders or quarters, others have to. If you want to go with 17's and that kind of drop, use this link to help find the proper size tire:
http://www.miata.net/garage/tirecalc.html
Input the factory size (225/50/16) and start messing with sizes in the other side. you want to find one close to the diameter of the factory tire. This will also keep your speedo within an acceptable range of correct.
FWIW I run 17x9 +20 fronts with 245/40/17 and 17x10.5 +24 rears with 275/40/17 I'm currentley running factory struts with Eibach lowering springs. I do not rub, or scrape anything.