Such right-of-way and parkway shall be of such size and dimension as may be required by the United States government or its subsidiaries in charge of any such parkway road, not exceeding, however, one hundred (100) acres on the average to the mile of such roadway in the State of Mississippi, and such scenic easements in addition thereto as may be required, not exceeding, however, the amount of fifty (50) acres on the average to the mile of such roadway in the State of Mississippi.