Wim,
The standard height is 19.5 centimeters and the standard lenght is 39 centimeters. The width varies. You can purchase blocks in widths of 10, 15, 20 and 30 centimeters. The standard measures are in inches, so when you go to buy them you should ask for blocks of 4, 6, 8 or 12 inches. Some factories now also offer a 5 inch block.
Price varies depending on where you need the blocks delivered to. Currently the price of a 15 centimeter block is $9.75 pesos. This price does not include sales tax or delivery charge. You can find cheaper blocks in the market, but due to their extremely poor quality I wouldn't recommend them.
PM me if you need more information regarding construction materials in the DR.
Jojocho