A 225/55 15" is the closest diameter match to your stock size, but may be a bit wide for the factory 7" wide rim? The 205/55 won't be any wider than a 205/60, just a shorter sidewall.
This site http://www.miata.net/garage/tirecalc.html
lets you play around with tire size effects and get instant answers.
If you want more rubber on the road, and can spend the dollars, get the MB 6 spoke sport wheels in 16" diameter and install 215/50 or 225/50 rubber. It is less than 1% difference in diameter either way, and looks great (see below for my 98 C230 with 205/55 tires, probably 215 0r 225 next time).
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E