The minecraft world is about 3 million blocks across (or 6 mil)
Ingame, if you type /tp [username] 0 0 3000000 then press enter, it should take you to that location
Walk around for a little bit in a BIG circle and if you fall out of the world, you found the edge of the world. That is when you find the area that notch made the blocks look as if they are accually there, but they are not.
In terms of surface area a Minecraft world is about 7 times larger than Earth; 60000 x 60000 (1 block = 1 meter) = 3.6 billion cubic kilometers; the surface area of Earth is 510 million square kilometers, which is about 1/7 the area.
However, the Earth is vastly larger in terms of volume, at about 1 trillion cubic kilometers as opposed to a mere 921.6 million for Minecraft, making the Earth 1085 times larger, since the Minecraft world is only 256 blocks/meters high; even if you include all dimensions, which are all the same size (arguably the Nether is only half as high since while you can go up to y=256 the world never generates above y=128), the volume of Earth is still 361.7 times larger (434 with the Nether being 128 high).
Also, a fully generated world will require about 64 petabytes of storage, based on an average of about 5 KB per chunk (from a world generated in 1.6.4 which takes up 525 MB and has 109595 chunks (Overworld only) ; newer versions may be slightly larger due to more complex world generation; a modded world that I have averages 7 KB per chunk, conversely, these values are higher than a freshly generated world since by lighting up all caves the complexity increases, increasing the size of chunks despite no actual increase in the amount of uncompressed data stored as light levels are always saved for all blocks).
If uncompressed, a fully generated world with an average height of 5 chunk sections (between y=64 and y=79) will take up about 680 petabytes, using the in-memory storage format used by 1.6.4 (newer versions are more memory intensive due to how they store blocks; for every chunk section 1.6.4 needs 4096 bytes for block IDs, 2048 bytes for metadata, 2048 bytes for block light, and 2048 bytes for sky light. Each chunk in turn has 2048 bytes for heightmaps, 256 bytes for biomes, and an estimated 1024 additional bytes for various other data, including entities. For perspective, this amounts to about 440 MB on the client and server (each chunk is loaded twice) at a render distance of 32; for a loaded height of y=128 (Nether) it requires about 674 MB, and 1.3 GB for a full-height world).
Note that the compressed size is actually smaller than suggested by the size of region files due to the way the game stores chunks within region files; it aligns them to 4 KB "sectors", much as a file system does, so there will be empty space between chunks (a lot in the case of a void world, which will still need 4104 KB per region file (4 KB * 1024 chunks + 8 KB header) even though they are mostly empty, as seen when you compress the file, which eliminates the empty space). This also exaggerates the size increase when chunks become slightly larger than 4 KB, as they now take up 8 KB on disk.