I use a 2 x 10 for my ramps (2500 lb car). And mine are easily 10 or so long.
But,,,,remember, wood is bendy on the flat, but stiffer on the edge. So I got (2) 2x4s for each ramp and glue (Gorilla glue) and screwed them to the side of the 2x10s. Like a C channel on it's side. Or an upside down U, with shot legs. I use hinges with looong screws that slip into bars on the trailer to lock them at the trailer, and shaved down the section at the back to mate well with the ground. I added non slip tape for traction. They tale 30 seconds to set up or break down, and you don't have the issue of supports that fall out or are the wrong length for the terrain you are on to deal with.
Result is a pretty long but light ramp, 12" wide, dirt cheap and easy to build, and has lasted years with no issues. Be selective when buying the lumber, and get the straightest tightest grain you can.