I came up with an estimate of roughly 200,000 dwarves, based on:
12x12 site * 48x48 tiles in a region tile * 150 z-levels = about 50 million tiles in a large site
I once rather comfortably accommodated a 200-dwarf fort in what were probably about 6 main z-levels of a 2x2 region-tile area (the rest of the site was just mines, waterworks, etc). This was with a lot of 2x2 bedrooms with chests and cabinets unoccupied because it was my first time playing with the economy, so I ended up digging a lot of extra housing for the poor.
2x2 * 48x48 * 6 = on the order of 50 thousand tiles.
If a site with a thousand times more tiles can accommodate a thousand times more dwarves, a full 12x12 site in 0.31 should be able to accommodate two hundred thousand dwarves if we neglect the computing resources that would be required for that.
So the obvious next questions to ask are:
Has anyone ever measured how much food and drink a dwarf consumes in a year, how much a farm tile can produce in a year, and the rates at which dwarves can carry out all of the farming related jobs?
If a fort had a population of 200,000 dwarves, how many tiles of farm plots would it need, and how many dwarves to work them? (considering planters, threshers, millers, brewers, cooks, and probably a lot of full-time dedicated food haulers assigned to a number of burrows controlling food distribution)
How many tiles of the map would need to be dedicated to food stockpiles, and what would be the most efficient arrangement of them? How many barrels should there be? How many of each kind of workshop, and how much power will be needed for millstones?
Once we have the food industry taken care of, a small number of dwarves will be needed to make things like furniture and weapons needed within the fort. The fort would need huge numbers of beds and doors and the like, but only once, so we probably don't need more than 1-2 hundred each of masons, carpenters, blacksmiths, glassmakers, etc (any more than a normally-sized fort needs more than one legendary mason).
That leaves the remainder of the fort to be split between the military and wealth production for export. So the last question I have is:
Arbitrarily approximating that we'd need no more than a thousand soldiers and could use all other dwarves to make, decorate, and haul items for trade, how much wealth per year could the 200,000-dwarf fort be expected to export?