30 is fun when concentrating on the personal aspects. I can remember most dwarves by name, profession and preferences and the game is more like "the sims". Having no real attacks makes it somewhat boring and building anything needs years, even when everybody is legendary in their profession.
Around 60 is great for a micromanaged fort. 15-20 can be specialized workers, 24 make a decent military and the rest can do hauling and masonry without anyone ever having no job. Megaprojects take a lot of time, though.
100 makes for a good military fort. 40 can do the supporting work with very few skilled workers for making weapons and armor. Having 60 or more dwarves in the military makes battles more interesting, because you can actually manage your squads and not just order everyone to charge into goblin christmas.
For megaprojects, more is better. I don't know if my system can handle 200 or more dwarves, but if it works, I'll have them all. Send 10 miners to the quarry and 80 masons to the building site... no one cares about their names or exact numbers, as long as the work gets done. A dozen workers died in an attack? No problem, new ones are on the way.
Of course, children are a problem... of 139 dwarves in Artobducim, 53 are children, and there's barely a month without another one popping out. Most of the mothers are in the military, so killing them would basically result in enraged legendary hammerdwarves playing golf with civilian's heads. In 12 years, it'll be awesome, but now it's just annoying.