The way it was explained when we took our PTC class, if you see the sign, even if it's not legal size etc, it is advised to see the point they are trying to make. On the flip side, if you're concerned about people trolling the parking lots looking for people unloading 'valuables' into their vehicles, a handgun in your case.. Yes, you can be asked to leave if they see your firearm and choose to confront you. Worst case, if you refuse to leave, they may call the police and you'll be written a request for a $25.00 donation.
Personally, I haven't encountered the situation yet, and if patronizing a new place, I'll troll past the front door if possible to look for a sign banning them or not.
Now, for the sign to be 'legal' is must be of a specific size, 187 sq inches if I recall, and opposing color block letter/background. but again, the their point is being made with any signage, I'll opt to not be one to cause problems etc, so I'll go elsewhere in most cases..