Our world has been designed around a concept of an (almost) infinite world. The actual size will vary, as we will experiment with chunk sizes. But as it stands at the moment, the grid is capable of number of chunks in all directions. Each chunk is blocks large.
This gives us a theoretical world size of in any direction.
In volume this is cubic meters. In cubic kilometers this is 4,056481920730334e+22.
This is larger than the earth. The earth has a volume of “only” cubic kilometers.
Our game is much bigger than the earth!
In fact, it’s larger than the sun too (with a volume of 1.412*10^18 cubic kilometers).