The largest tire that can be installed on a stock Honda Pioneer 1000 will depend on the specific model and year of production, as well as the vehicle's suspension and other components. In general, the Honda Pioneer 1000 is equipped with a suspension system that is designed to accommodate larger tires, and it is possible to install tires that are up to 28 inches in diameter on some models without making any additional modifications. However, it is important to note that installing larger tires can affect the vehicle's performance and handling, and it may be necessary to make additional modifications to the suspension or other components to ensure that the vehicle is able to accommodate the larger tires safely.