Here’s some good advice from “Chris” at Signwarehouse.com you can pass on to your customers . . .
“Don’t use standard magnetic signs on the sides of vans or box trucks; there is a reason they make vehicle size graphics. A size 12 x 24 magnetic graphic looks ridiculously small on the side of a van because it takes up just a fraction of the available space! The negative space is all wrong. Vehicle graphics, designed large to fill the space on the side of the truck, will attract much more attention than the little magnetic graphics. If you do use a car for dual purposes, such as for business and family use, you might employ small magnetic graphics on the doors.”