I use the 2x2 trays. I clean poop off the litter, but when I have to change the whole deal with new pellets, I pick the whole tray up and carry it out to the wheel barrow. The bigger ones are pretty heavy to lift when they are heavy with urine.
You can also dump the 2 x 2 trays into a large garbage bag to empty. Again, hard to do with the bigger trays.
I have also found that I need to put 2 of them side by side, though. For whatever reason, when I tried only one, the puppies just could not seem to go in the tray better than about 50% of the time. When I put two of them next to each other, almost perfect.
The size I use depends on how big the litter is. Sometimes I have two side by side. I think if the pups have plenty of room they are more likely to use the trays. If the trays become filled with with poop and pee after a long day or if the pups are getting bigger and there is less room than in the beginning then they will not stay as clean or want to use the area outside the trays.
I've used both the 2x2 and the 3x3. Depends on where the puppies are, whelping box v. kennel run.
Mike is right, a single 2x2 is easier to manage for cleaning but a bit small for the puppies aim. With the 2x2 I did have puppies that just couldn't aim their little rears inside the box all of the time.
A 3x3 can be slid into a garbage bag for dumping but it needs to be a bigger bag than a 30gallon.
I use a large under-the-bed storage box. I remove the 2 wheels and handles, cut a door in one side with a hand coping saw and use carefresh bedding (pine shavings or pellets work too). It's long and narrow so it can be dumped in a 30 gallon can easily. Save the lid and store left over supplies in the box until next time. Inexpensive < $20.
Found something different to use...trays for hot water tanks. trays are made of rubber or metal..about 24" round..Found at Home Depot in the plumbing department..Rubber trays are about $11. Very easy to empty and clean. Light weight.
