A small update, no pun intended. The theoretical size have changed to 2^64 chunks in each direction. A bit larger world in other words.
1 chunk is 8*8*8 meters. That leaves us with a distance of 2^67 meters per axis. Translated to astronomical speak, this is about 4.8 kiloparsecs. The diameter of the Milky way is around 31-37 kparsecs.
The Milky Way galaxy is 10 times too fat for Monster, but still. Monster is wayyy larger than Minecraft ;).