Wheels
18" wheels look great and IMHO the largest diameter to go. Finding tires is a little trickier and quite a bit more expensive.
19" wheels look good to, but start to make the OE brakes look pretty tiny, plus finding tires is pretty tough.
Width:
9"(min) to 9.5"(max) wide in front with 275/40ZR17 or 275/35ZR18 tires.
10" to 11"(max) wide in back with 315/35ZR17 or 295/35ZR18 tires.
The 17" tire combo will not mess up the speedo, but 18" wheel-tire combo with just slightly mess up the speedo, but can be corrected.
So ultimately I think price will be the major descision maker for you, look into the prce of wheels and tires and get what you can afford. Hope that helps.
See sig for what I am running on my red convertible SS.

