Nonresidential building wall signs shall be permitted, provided such signs are approved as part of a comprehensive sign plan for each development. Except for buildings containing multiple retail and service business uses, no more than two (2) wall signs shall be permitted for each building and no individual wall sign shall exceed an area of five (5%) percent of the building facade to which it is attached. Buildings containing multiple retail and service business uses may be permitted to have a wall signage system that also allows each retail and service business use to have an exterior building sign, but all such signs must be part of a coordinated sign plan (as to color choices, lettering style and size and sign placement) approved for the development by the Land Use Board at the time of site plan approval, and no individual retail or service business wall sign shall exceed a maximum area of twenty (20) square feet nor a maximum vertical sign dimension of two (2) feet.