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.
Built these this weekend 17"x12" screwed together cost me about $45 and took about a 1/2 hr for each one. Which seemed like a long time but it was a lot of screws