Always measure the plastic. Guessing is risky.
Also, large faces on many better-built signs will have hanging bars. You don't want a hanging bar to be a surprise when you are ready to install just because you didn't want to take the time to pull retainers when surveying.
In addition, if the sign was built the traditional way (angle iron frame with an aluminum skin), dimensions can vary considerably from standard "knock-down" kit dimensions. Lamp lengths have not always determined sign dimensions. For example, cabinets that were built to fit between existing poles may have been sized to fit the space exactly. Lamps were then fitted to the cabinet by adjusting the height of hat sections or even overlapping lamps. When overlapping lamps, you can put an opaque lamp sleeve over one of the lamps at the overlap to prevent a "hot spot."
If U-lamps were used, a cabinet size can vary for that reason as well.
Obviously, a custom-shaped face should be brought back to the shop to use as a template.
Brad
Cold in Kansas City