This is a state-specific question. In Ohio, there are guidelines for the size of the sign, font size, exactly what info the sign must contain, and where the signs are to be located.
A quick Google search shows FL law has similar requirements:
715.07
a. The notice must be prominently placed at each driveway access or curb cut allowing vehicular access to the property, within 5 feet from the public right-of-way line. If there are no curbs or access barriers, the signs must be posted not less than one sign for each 25 feet of lot frontage.
b. The notice must clearly indicate, in not less than 2-inch high, light-reflective letters on a contrasting background, that unauthorized vehicles will be towed away at the owner’s expense. The words “tow-away zone” must be included on the sign in not less than 4-inch high letters.
c. The notice must also provide the name and current telephone number of the person or firm towing or removing the vehicles or vessels.
d. The sign structure containing the required notices must be permanently installed with the words “tow-away zone” not less than 3 feet and not more than 6 feet above ground level and must be continuously maintained on the property for not less than 24 hours prior to the towing or removal of any vehicles or vessels.
e. The local government may require permitting and inspection of these signs prior to any towing or removal of vehicles or vessels being authorized.
f. A business with 20 or fewer parking spaces satisfies the notice requirements of this subparagraph by prominently displaying a sign stating “Reserved Parking for Customers Only Unauthorized Vehicles or Vessels Will be Towed Away At the Owner’s Expense” in not less than 4-inch high, light-reflective letters on a contrasting background.
g. A property owner towing or removing vessels from real property must post notice, consistent with the requirements in sub-subparagraphs a.-f., which apply to vehicles, that unauthorized vehicles or vessels will be towed away at the owner’s expense.
