Those wheels are 20x8.5 inches. 8.5 inches should accept really wide tires (at least 275 or 285).
Call an expert at tirerack.com (etc) to find out what the widest size you can fit is. With a 20 inch wheel, you are going to have to use *extremely* low profile tires to maintain the same rolling diameter.
Finally, please weigh your wheels when you get them. I’m curious how much 20” wheels weigh. I've spent hours searching for wheels online and the dam dealers rarely tell you how much they weigh. Some people (like me) care about this stuff.