Hello, we do vehicle wraps and in Ohio on trucks we usually size the ODOT numbers at 2.5" tall and they strech symmetricaly. that is the maxi,um size we have done. I don't beleive there is a specific but you should call DOT in your state. I looked on web and have seen between 1.5 and 2.5" When trucks come to us with them already that is typical. rick