The gross area of a tract of land proposed to be developed in a RPUD district shall be a minimum of 100 acres, provided however, that smaller parcels may be considered on the basis of their potential to satisfy the intent of these regulations. A minimum of twenty (20) percent of the land developed in a RPUD project shall be reserved for common open space and recreational facilities for the residents or users of the area being developed. The common open space shall be disposed of as required in Section 1113.07 of this Ordinance.