The size of the oil filter used in a Harley-Davidson motorcycle can vary depending on the specific model and year. Harley-Davidson motorcycles typically use a spin-on type oil filter. The most common oil filter size for Harley-Davidson motorcycles is 3/4"-16, which refers to the thread size and pitch. However, it's always recommended to consult the owner's manual or refer to the specific model's documentation to ensure you have the correct oil filter size for your particular Harley-Davidson motorcycle.