Canoetrpr,
on our farm the previous owner already built some pallet dollies with the same size wheels you showed. For the rototiller I used bigger wheels becasue it is heavier.
The bigger wheels seem to roll easier than the smaller wheels. But the same size wheels you showed work just fine. Just oil them, or perhaps somebody here knows a better product to put on the ball bearings other than oil, but that is what I saw on the workbench so that is what I used, and if using the smaller wheels do use more than 4.
Attached is a picture of me putting on a wheel, this shows the bigger wheels I used for the heavy rototiller pallet. Actually my husband said it sits up jsut a bit high for him now when he attaches it and if I was to do it over again, I would have used the smaller wheels.