I have some questions for all you engineers of awesome frotressi. I see these massive above ground super forts and i was wondering how you assign so much space? seems like 90% would be pointless and empty.
Extravagance is the name of the game on many of these forts. 3x3 bedrooms for commoners, all 160 of them! 13x13, 3-level high bedrooms for the nobility and the most useful legendary dwarves! Multistory dining halls! 31x31 stockpiles, for masterwork pieces only! Several grand, multistory statue gardens!
...Buy yes, much of many of them are just things being big for the sake of being big. Lots of wasted space for aesthetics and grandeur.
second question. do you build temp forts to house your people while you build it?
That's a difficult question... How temporary is a temporary fort? Many people build a complete, full-fledged, totally-self-sufficient fort that has all the amenities that any fort would have, because that fort was the original goal. Then, once they've got such a fort that no goblin could ever breach its walls, they go for the megaprojects to stave off boredom. So technically yes, they do, but the "temporary" fort is actually a permanent fort that is abandoned (in the sense of "people don't use it anymore") when a second, better permanent fort is built on the same site.
Other times, when the megaproject is the actual goal of the embark, it depends on what the project is. A glass pyramid megaproject, for instance, is likely to have sand and lava available from the beginning, so it might be practical to just get living spaces in the pyramid's lower levels built rather than making a temporary fort. On a magma-casting project, though, it will take so long for even the first level of the final project to be finished that it simply isn't feasible to have your dwarves live in the wilderness until then.
So, in other words... It depends on the project.