In all of the promotional videos they mentioned it will fit a full size pallet at least 6/7 times. I'm not sure how much difference there is between the P3 & P5 on bed size.
The 1000-3 seems quite a bit larger, wider for sure. I believe at the tailgate the 1000-5 and 700-4 are nearly identical based upon part numbers but the 1000-5 is wider by a few inches behind the cab. In conclusion my observation of them side by side is the box is maybe about 15-20% bigger on the 1000-5 over the 700-4.