Size of pad - it really depends upon the surface. Some places you really need to spread the load a lot. I have 12x12 pads, and have had them sink into the ground a few times where I had to park on grass and it rained. When I think it might be really soft - I use some 2x8 boards that I have cut into 12 inch lengths. Not because they are less likely to sink in - but because I have handles on them and can pull them out with the claw of a hammer.
Between the wheel chocks - I love them. They are easy to adjust and setup. And besides doing a very good job of making sure the trailer cannot roll - they also help reduce movement as we walk around the trailer. Our last trailer - the
BAL X-Chocks had to be open to about 8 inches to snug between the tires. The new trailer they are almost closed - about 2.25 inches between the tires.