The structural calculations for signs along roads vary in complication, depending the cases and applicable codes. There are even some recommendated specs in the UK that ask for the consideration of aerodynamical effects, which highly complicates the matter from the analysis viewpoint.
Try a search in Florida FDOT for some freely available programs developed to the purpose.
I would suggest that you start with ASCE7, which specifies minimum loads for structures, signs included. You will have to know the location in order to determine the required wind velocity, but you can get the required forces there. You will then have to check the sign (assuming it is existing) based on the standards for the materials used in the sign and also check the foundation.
I came across a specialist sign engineer in texas that crunched out basic sign designs all over the country for only a few hundred bucks, I suggest you look them up as it looks like you are out of your comfort zone.