The diameter of the wheel plays a less important aspect as long as the wheel clears the strut and the caliper up front (more room for the rear). The next step is sizing the tire, depending on several aspects, the tire OD can vary as much as an inch or so from the original OD which is just about 26 inches.
The answer to the question whether a 20" wheel would fit your Stinger is it depends on the wheel and its specs.