proper term is cribbing blocks. They use these to lift houses from their foundations.
Heres a great write up to make a sturdy set.
http://www.myturbodiesel.com/1000q/multi/wood-block.htm
Tire size will determin width and length. I have found that 16" long and wide works well for tires up to 345 mm. I like to use 2" wood screws in case I want to lower or increase the height easily.