I think that's one of those laws that took on a life of it's own through word of mouth. I looked through Virginia's motor vehicle laws and could only find reference to load width limits: (The only reference to height limits I could find was 12 feet.)
§ 46.2-1111. Extension of loads beyond line of fender or body.
No vehicle shall carry any load extending more than six inches beyond the line of the fender or body. Nor shall such load exceed a total outside width as prescribed by §§ 46.2-1105 through 46.2-1109.
Notwithstanding the foregoing provisions of this section, watercraft carried on vehicles may extend more than six inches beyond the line of the fender or body of such vehicle if the total width of watercraft and the carrier upon which it is carried does not exceed seventy-six inches.
(Code 1950, § 46-330; 1958, c. 541, § 46.1-334; 1960, c. 280; 1979, c. 526; 1989, c. 727.)