Pine boards are typically 1 inch thick. They're usually cut almost square, but not quite, because you want to make sure you don't accidentally line them up cross-grained. My instructor usually brings us boards that are cut 12" x 11" X 1".
Rank isn't usually an issue in board breaks. The confidence of the breaker in his own skill is what determines how many boards he or she breaks.
Now, despite that, some styles require board breaks for promotions. To become a 1st dan, I was required to break 3 boards with any one technique of my choice (I chose sidekick). To be promoted to 2nd dan (which is a ways off for me) I'll have to break 4 boards twice, once with a hand technique and once with a foot technique (4 is no problem for my sidekick, I need to work on my punch or forearm smash a bit).
Some people may choose thicker boards or denser substances for fancy trick breaks, but I've never heard of that stuff being required for promotions.
Anything else?