yeah. basically as big as u want. but u will need to replace some suspension parts the bigger you go (raise the car for 30" rims)
if u keep stock components, u can go 19 and 20. if u do 2" lowering. u can still do 19. it all depends. a good size is 18" with tires that keep the same wheel diameter as stock. check http://www.miata.net/garage/tirecalc.html for that.
but yeah. read thru the first link terra gave u. it gives good info