Generally you should measure your vehicle before browsing the website for a specific product. Using a tape measure, yard stick or other device, define the area where the decal is to be installed (windshield, side window, rear window, wing, gas tank-motorcycle, etc.) Measure with height (up and down) and width (left to right). These numbers should be in inches.
Be sure to make the decal smaller than the application surface. This will make the installation easier, and the decal will fit the area properly. For example: Most windshield decals are 4in tall by 40in wide. The actual windshield area is normally about 46in to 52 in wide, but the decal should be smaller than the application area.
A NOTE ABOUT COLOR: At the same time, look at the application area and visualize the decal, and choose a color or colors for the decal. Generally a light colored decal will look best on a window or dark painted surface, while darker colors will look best on lighter surfaces.