Most likely it's 48x40, standard pallet size. It's is a skid, used when a pallet is unnecessary but you still want to be able move whatever with a fork. This thing probably came from a factory that produces something big, rectangular, and relatively light. They probably put these on trucks (48x40 pallets fill American trailers most efficiently) and move the product to another facility. Once every few weeks or months they'll fill a truck with a bunch of these skids and return them to the factory to start the process all over again. This is why you find them, unless you know someone that moves or loads them. They normally have the owner stamped on them and something about theft and the law. I'd like a few!