What are the rules exactly? Let's say you are leasing a semi, delivery van or courier vehicle for your logistics business. Since the truck is not something you personally own (or will ever own for that matter), you would like to limit the amount of alterations you make to the vehicle to insure a smooth transition when the terms of the lease expire. You also have to identify your vehicle with the proper GVW, MC, ICC or US DOT Numbers in order to be compliant with the DOT. According to the US DOT publication ETA 501c section 390.21 referencing vehicle identification the rule is this:

"Every commercial motor vehicle operated by a motor carrier in interstate commerce must be marked,

on BOTH sides of vehicle, with the following: 
➤ The motor carrier’s name or trade name ➤ The motor carrier’s identification 
number, preceded by “US DOT” 
➤ The size, shape and color markings must be in letters that contrast sharply in color with the background on which the letters are placed 
➤ The lettering must be readily legible during daylight hours, from a distance of 50 feet."

* While it is true that the law does not specifically state a size or color requirement, it does say that it needs to be "legible" from 50 feet. The sign industry standard for block lettering to be read at 50 feet is 2" high. Other fonts and colors are perfectly legal, they just need to be sized a little larger to be read from a 50 foot distance.