6 inch wheels are what we have gone with for the last 2 years. We like them because they allow for climbing easily, and are generally nice to design into as frame. As for what we use for the wheels, we had our own made by our CNC sponsor. you can see them here: (http://www.chiefdelphi.com/media/photos/22758)

One of the main reasons we pick this size over others is the fact that we've used it many times before, and it has been reliable and consistent. We perfer to stick with something we know works, and only go out on a limb for a new design occasionally.