To be USDOT & FMCSA compliant your USDOT lettering must be 2 inches in height or larger, must be visible from a distance of 50 feet away. Also, your lettering must contrast with the color of vehicle for easy readability. The font you use also matters, the font must be easy to read and you should primarily use block style font type. You don’t want to give any reason to the DOT inspectors to pull you over and potentially put you out of service or even give you a fine. You want to display all of your crucial information neatly and professionally outside of your commercial vehicle.