Calculating 20 Unbelievably agile dwarfs carrying one building material at a time, and installing it, from only 20 squares away, at a nicely impossible 30fps, you're looking at... approximately 70 hours of playing. Enjoy your 1million-block megastructure! Anyone care to calculate how many z-levels of a reasonable structure footprint that would fill, solid? Now, if carts were implemented to move large amounts of materials, perhaps it would be possible. But now? bah.
If anyone does that, I'll feel totally emasculated as a dwarf fortress player.
First you'd need a million stone. Disregarding soil layers, features, the fact that some of these will be gems and metals, and mining skill, on a perfectly flat map you'd get:
1x1 embark: (48-2)(48-2)(15) = 31,740 stone
6x6 embark: (6*48-2)(6*48-2)(15) = 1,226,940 stone
11x11 embark: (11*48-2)(11*48-2)(15) = 4,150,140 stone
16x16 embark: (16*48-2)(16*48-2)(15) = 8,801,340 stone
So the smallest embark you'd need to get a million stone would be a 5x6:
5x6 embark: (5*48-2)(6*48-2)(15) = 1,021,020 stone
Now that you've gotten the stone, you need to build something out of it. You could make a 100x100x100 solid cube out of exactly 1 million blocks, if you had a map that had at least 100z's and was at least a 3x3 embark, so you'd have enough buildable space to assemble it.
Made the cube? Want to see what its worth?
Made of raw wood (worth ☼3): 3,000,000☼
Made of any non flux raw stone (worth ☼3): 3,000,000☼
Made of any flux raw stone (worth ☼6): 6,000,000☼
Made of raw obsidian (worth ☼9): 9,000,000☼
Made of wood blocks (worth ☼5): 5,000,000☼
Made of any non flux stone blocks (worth ☼5): 5,000,000☼
Made of any flux stone blocks (worth ☼10): 10,000,000☼
Made of obsidian blocks (worth ☼15): 15,000,000☼
Made of iron blocks (worth ☼50): 50,000,000☼
Made of gold blocks (worth ☼150): 150,000,000☼
Made of aluminum blocks (worth ☼200): 200,000,000☼
Made of dragon soap blocks (worth ☼1,250): 1,250,000,000☼
Made of adamantine blocks (worth ☼1,500): 1,500,000,000☼
To put it in perspective, 1 million blocks would be enough to build
FlareChannel 4 times over.