16x16 blocks square, and includes everything (128 blocks) from top to bottom limits.
And for reference, the game loads into memory the 9x9 grid of chunks around you (81 chunks, or ~21k blocks square, or ~2.7 million blocks in volume), but only the 4x4 chunks centered around you (16 chunks, or ~4100 blocks square, or ~500k blocks in volume) will be going through their scripts and processing activity (e.g. growing trees, mob spawning, etc.).
This is from the Minecraft wiki.
XI Wiki




