#1 I hope that was a slip of the tongue and/or fingers when you wrote LEVELING jack. They are not for leveling, they are only for stablizing the trailer. You level with your wheels and tongue jack.
#2 I usually do 2x6x12 boards. That is plenty large enough so if you put it on softer ground, it makes a bigger footprint so you are less likely to bury your blocking. Also, as was mention, the blocks then become long enough to use under tires if need be.
Happy camping! Our next trip is 3 weeks from yesterday... nice trip to FLORIDA in February. Hope it heats up before we get down there though!!!