There are 2 common dimensions for yoga blocks:
Size 1 - 3" x 6" x 9"
Size 2 - 4" x 6" x 9"
Initially yoga blocks were made of wood (typically bamboo, poplar, birch or balsa), but today you can find blocks made of hard foam rubber and cork. The material that the block is made of will change their weight, durability, rigidity, and price. Wood blocks are comparatively hard and heavy but are very sturdy, and can hold body weight for support. The foam rubber blocks are softer and lighter but may compress when your full body weight is on them. Cork blocks are closer to wood blocks, but tend to be less slippery.
If you do not have blocks you can improvise by stacking books.