The bigger, the better. Use treated lumber.
I was fortunate about 15 years ago to get some timbers from my in-laws barn. They took a wall down, and I got a couple of the post. The posts were a true 6 inches by 4 inches, and he wood was probably over a hundred years old. To cut them to length I had to use a chain saw because the wood was so solid. I have 6 blocks I used under the jacks. I used them unpainted for years, but a couple years ago, decided to paint them so they look a little nicer. These are the only things I use. Sometimes, if the ground is really unlevel, I'll put a 2x8 under the block. The 2x8 is about 2 feet long. I have several of them. These are heavy, but I don't want to use anything else.
This is not a good close up of them, but you can see them here under the jacks (those orange blocks). The rear of the camper has 2 stacked on top of each other. One reason why I like to use these, I can turn them on their side, and it make the distance from the bottom of the jack to the top of the block shorter, giving the camper more stability because the jacks are not extended so much.
If you are wanting a size, I suggest using 2x8x18 inches as minimum, and use treated lumber. 2x12x18 is even better. The bigger footprint, the less the block will sink (especially if you park on grass... which we do a lot).
