I'm not sure you got your math right there. A pocket world is 17x17, and each of those world tiles contains 16x16 embark tiles. A massive world should be 257x257 and then 16x16, you added another 16x16 in there that didn't belong.
Fun fact: An embark tile is 48x48. A world tile is 16x16 embark tiles. A maximum size world is 257x257 world tiles. That's 2,304 unit tiles per embark tiles. 589,824 unit tiles per world tiles. 38,957,285,376 unit tiles in a maximum size world. The longest straight path would be 197,376 tiles long. At average speed of 9.5 ticks per tiles, for your average dwarf, it would take 1,875,072 ticks to cross the distance. In fortress mode, a day is 1,200 ticks, so it would take 4.3-ish years of non-stop walking to cross the world. I can't find the exact length of a day in adventure mode right now, but I know that it's over 10x longer than a fort mode day.
To take this further, the circumference of the Earth is approximately 40030.2 km. If we take the average DF unit tile to be a cube 3m on a side, this means that it would take 13343400 unit tiles to stretch around the equator. At the aforementioned speed of 9.5 ticks/tile, a fortress dwarf could walk the equator (without stopping, ever) in 126,762,300 ticks, or almost 290 years. A human with a walking speed of 5km/hr could walk the same distance (again, without ever stopping) in about 22 years.
The surface area of all the land on Earth totals around 148,940,000 km
2. Again, assuming that a DF tile is 9 m
2, it would take 1,654,900,000,000 DF tiles to cover all of Earth's land. If 40% of this is hospitable, then that leaves a total of 661,960,000,000 DF unit tiles that can sustain a fortress. This means that, at maximum, there could be 718,267,747 2x2 fortresses on Earth. Dividing the current human population of Earth between all these fortresses would leave each fortress with fewer than ten people.
Going the other way yields even more interesting results.
