Most people don't like to go any wider than their tow vehicle because doing so requires wider side view mirrors.
Beyond that, if you want to get specific, search for cantilevered beam formulas or on line calculators; they will require you to enter the correct data for your application, including 'live' and 'dead' loads and material specific properties. You will have to decide what factor to add for dynamic loading, as these calculators will assume a static structure, not one that is bouncing down the road. Remember: GIGO (garbage in = garbage out), so the more accurate your specs and assumptions, the more accurate the results will be.
... and now you know just enough to be dangerous!!!