this is what i could find:
posted a sign that is located in a prominent place near all
of the entrances to the part of the building to which the
restriction applies and any individual entering the building
can be reasonably expected to see the sign.
here's some more:
943.13 (2) (bm) 1. In this paragraph, “sign” means a
sign that states a restriction imposed under subd. 2. that
is at least 5 inches by 7 inches.