I use two, 2" x 10" x 4 feet. Cut the ends at 45 degrees (so, they don't move when I drive up on them). I have marked the the boards to match the centers of the wheels DW tells me when we are centered. Also use them for leveling when needed.
2015 Flagstaff 8528IKWS Diamond Package
2013 Ford F-150 Super Crew, EcoBoost, Max Tow,
6.5 bed, Roadmaster Active Suspension, E rated tires
91 days camped 2015
15 days camped 2016 more planned