Minecraft's world is made up of chunks, which are 16 x 16 blocks large on the X and Z axis, and 384 blocks on the Y axis. When you first load into a world, a set number of chunks load around you, ...
Every world in Minecraft is pretty much infinite. Each world is divided into chunks that are 16 blocks wide, 256 blocks tall, and 16 blocks long. When fully filled, a chunk has 65,536 blocks in total.