Truck size is measured by axle width or hanger width. The width of the hanger and axle determines how far apart your wheels are. The width of your trucks will affect the performance of your board as different truck preparations are better suited to certain riding styles. The most common setup is that the truck axle is about the same width as your deck - this setup offers the most stability. Generally, go with a shaft that is equal, larger or smaller than the 1/4 ″ width of the board.