Mine are 3/4 CDX (one sheet per horse), but they are glued up, double thickness, so about the same thickness as 2x and with rails about 12" deep. Very strong. The engineer I am working with thinks we can put a truck on them, though we have not done the load testing yet.how much weight could those really hold? we build our horses out of 2x4 or 2x6 but only make them to cut rafters on and load them up pretty heavy.
We build our horses out of pieces of wood I . you can make them as long as you need them. We generally make them about five feet long. You nail approx. 9 inch 2x4 to underside and nail 32 inch legs up under top flange and to 9 inch piece. We found plywooding the ends made them extra strong. We set whole lifts of material on them with our telehandler and have not had any problems. Good advice to put blocks under legs if you are putting whole lifts on them, otherwise you may drive the legs through the subfloor.