From "Galaxy Size Comparison Chart" by Rhys Taylor. Get the Popular Science daily newsletter ... It’s puny compared to M87, an elliptical galaxy 980,000 light years in diameter.

And the 15 is the size of the wheel it's designed for. So, in this case you've got a 31-inch tall tire for a 15-inch wheel. You know the wheel radius is 7.5-inches, and the tire radius is 15.5 inches.