My boyfriend showed me this. Get your cock hard, take a Dollar Bill (or any 5, 10, 20, they're all the same size..) and wrap it length wise around the thickest part of your cock. Do the ends of the bill meet? My boyfriend has about a finger's width separating the ends.
I haven't tried it yet; but, I have tried the toilet paper roll test. I can't get it over my cock.