I purchased my ISSForged wheels for my SL55 from Rafic at Wheels Performance. He knew exactly what size wheels and tires would work on my car. In fact they are made specifically for the SL and sit perfect with the wheel wells. Take a look at my vehicle if you like. He has many companies to choose from but he knows what fits well.
I was going to get the 20" also but wanted a better ride so decided to go with the 19" myself. So glad I did. I have 285/30/19 in the rear and 255/35/19 in the front. The offset they use is perfect for the car. I have a 10" wheel in the back and a 9" in the front. The 19" wheel gives you a little more sidewall and provides a more comfortable ride. I lowered the car and to me cannot tell the difference between the two wheel sizes. My wheels never touch or rub. Just my .02$
You can do 20s, but as stevant says, the ride is somewhat compromised. Rough rodes are hell on them too. I live in the land of the giant pothole and refuse to change out my stocker 18s because even the 18s get tweaked with all of the rough roads in my area. I could not imagine what the roads would do to 20s. If you are wondering, yes, I miss as much rough roadway as I can. I even take different routes to places based on the roads. I was once pulled over by a policeman. He thought I was drunk. I was just dodging potholes. He was really cool about when I told him.