The average set of cabinets sold in home improvement stores varies between 8 and 10 feet in length, which means standard granite backsplashes are the same width, regardless of whether they are slab or tile material. The standard sizes sold in home improvement stores are averages, and there is no actual industry standard for countertop length, only averages depending on the type of home built.
The average slab size is the same as the length of countertops, and are specifically quarried that way to accommodate the average home. With an 8 to 10-foot slab it is possible to install a solid slab across the entire width of the kitchen backsplash, while for granite tiles you can generally run full 12-inch pieces from corner to corner in a 10-foot run, for example.
The type of granite you use can dictate the width of the backsplash if you are choosing to work with specialty granite or off-size slabs. For example, a 15-foot run is going to require two pieces of granite to be epoxied together, or for a specialty slab to be ordered. Other types, such as coral granite, may come in smaller, 3- or 4-foot slabs, which means you may want to adjust the width of your backsplash based around the slab itself rather than the cabinets.
The width of backsplashes comes down to personal preference every single time. There are no two kitchens that are exactly alike, which means every element is based upon your design choices and what you feel comfortable living with. While the average is 8 to 10 feet in width, you could choose to have a 20-foot wide kitchen or you could choose to have a kitchen that is only 6 feet in length based upon the type of wood used for the cabinets as well as the style and type of granite you choose to work with.