There's no easy answer to this question, because of the shape of the bulkhead at the front of the loadbed. I think most if not all Land-Rovers have a spare wheel mounting bracket fitted to the bulkhead which takes up some space, though it can be unbolted. and there may be seat belt mounting brackets fitted against the bulkhead and floor. If you were wanting to know the size of the largest rectangular box that would fit in the loadbed, then the length would be about 1023mm. If you removed the spare wheel mounting bracket then you would gain an extra 22mm. With the spare wheel carrier and seat belt mountings in place than the longest sheet of ply you could fit in flat on the floor would be about 1148mm