Large: (70″ length) is for all vehicle sizes, both 2 door and 4 door and typically covers front to back, wheel-well to wheel-well. To attach, you hold the panel aligned in position, press it on,  and the rubber-coated magnetic strips hold firmly in place. Small:  (46″ length) targets the sweet-spot area in the middle/side of vehicle. If unsure of best size; look at your vehicle’s design and measure the flat surface area to accommodate either the 70″x 10″ or 46″x 10″ panel.  (The two magnet strips on our panel’s backside, top & bottom edges, extend 10″ apart).