20 feet in length is the threshold for double charges.
The fees mentioned above are correct. The parking lots are operated by the Canaveral Port Authority (not Disney). They accept cash, VISA, MasterCard, and travelers' checks issued in U.S. Dollars. All fees are paid in advance when you park (e.g. $30 for a 3-day cruise, $40 for a 4-day cruise, $70 for a 7-day cruise.)