Yes as noted there are DOT requirements. Specifically address and related to physical dimensions, has nothing to do with weights, capacities or commercial-
http://www.nhtsa.gov/Cars/rules/standards/conspicuity/TBMpstr.htmlIf its over 80" & over wide the 3 center ID lights (visualy state presence of wide vehicle 80"+) and corner clearance lights ( visualy show width) are required on both the front and rear. However If truck cab has ID lights not required also on front of box though rear & corner marker lights still are.
As I read the requirements if under 80" the 3 ID (and outer top corner clearance/marker lights) are not required. Though Id still run the clearance lights just for visibility. Pop up not so much maybe but makes ensuring not hitting something a bit easier.