Pallet size would depend on whether they were homemade or made by Mann Lake or others. Here is Mann Lake's pallet size: 47" X 32.5" or 47" X 33.25". It depends on which pallet clip is used as the w clip takes more room side to side (33.25" side).
The other thing to think about is whether you will double stack, or only single stack them. You can get more pallets on a trailer by double stacking them. The real problem is you need to make sure the trailer will take the weight of the number of pallets you are putting on. Most single axle trailers are only rated for about 3500#, which includes the weight of the trailer. Double axle trailers are rated for 7000#.
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