In 1900, the original billboard standard was developed. The standardized structure was designed to hold posters of three (4 ft. x 8 ft.), eight (5 ft. x 11 ft.), or sixteen (10 ft. x 16 ft.) sheets. However, it proved to be anything but standard. Early frames were build with dimensional lumber, and could be any size at all. Later frames were covered with 4 x 8 sheets of plywood, so the dimensions tended to be 8,12, or 16 feet tall and 12, 20, 24, 32, or 36 feet wide. The actual "image" size is somewhat less, due to the required supports. Currently, the most popular sizes are (image size) 12 by 24 feet (30-sheet) and 5 by 11 feet (8-sheet).
So you will be accurate, no matter what size you choose, so long as it is reasonable.
In measuring some commercial HO billboards I have, the sizes are: 12 x 18, 12 x 20, 12 x 24, and 16 x 48.
Hope this helps,
Kevin
PS: Some billboard facts -
The first American billboard originated in New York in Jared Bell’s office where he printed posters for the circus in 1835.
The earliest recorded leasings of boards occurred in the U.S. in 1867.
By 1870, close to 300 small sign-painting and bill posting companies existed.
In 1872, the International Bill Posters’ Association of North America was formed in St. Louis.