Plus-size your wheels!

What is Plus Sizing, and its benefits?
I couldn't have said it any better than the people at Tire Rack:
"Plus sizing your wheels and tires is the best way to improve both the performance and appearance of your vehicle. By using a larger diameter wheel with a lower profile tire it's possible to properly maintain the overall diameter of the tire, keeping odometer and speedometer changes negligible. By using a tire with a shorter sidewall, you gain quickness in steering response and better lateral stability. The visual appeal is obvious, most wheels look better than the sidewall of the tire, so the more wheel and less sidewall there is, the better it looks."

Plus What?
Plus One means increasing the wheel diameter by 1", for example, going from 15" to 16".
Plus Two means increasing the wheel diameter by 2", for example, going from 15" to 17".
Plus Three means increasing the wheel diameter by 3", for example, going from 15" to 18".
... and so forth!

Close is good enough
Unfortunately, plus-sizing isn't an exact science. Side wall aspect ratios only come in multiples of 5, and tire widths are in multiples of 10mm. This makes it more difficult, because when you go to a lower profile tire, you may also have to get one that is wider in order for the overall diameter to be close to your OEM tires. Most of the time, the overall diameter won't be exactly the same, so the best you can hope for is "pretty darn close". If you're only a few tenths of an inch off, that's good enough.

Here is a Tire Size chart of all tire sizes that are readily available, and are within 1" or so in diameter of the reference size, 225/60-16 (which is the standard 16" tire size available on all W-body cars since 1991).

Watch out for rubbing!
When you go to wider tires than stock, you face the possibility of the tires rubbing on either plastic splash shields, or worse, metal parts of the wheel well and/or suspension. You will want to carefully measure tires for fitment, because most stores won't let you return wheels and tires after tires have been mounted.

Recommendations
Considering the size of W-body cars and the diameter of the OEM tires, for best looks I would recommend going with wheels that are 16" or larger in diameter, and tires at least 225-wide.
Sometimes people like to get that low-profile small sidewall look, so they put 40-series or smaller profile tires on 16- or 17-inch wheels. The problem with that, is the overall diameter will be smaller than OEM. As a result, there will be huge unsightly gaps in the wheel wells. The gap above the tire can always be eliminated by lowering the car, but then there are still the unsightly gaps to either side of the tire. The best way to get that low-profile look, is to use 40-series or smaller profile tires, but use a large wheel with a rim diameter of 18- to 20-inches. You will then get the low-profile look, and sufficiently fill out the wheel wells.