Hm some interesting notions there. Regarding the former, what would you suppose the limiting factor to be?
Yeah. Not sure on that one. It can't be spacedock: thats beam, not length or draft. It's probably not the shipyards, unless there's some distance they have trouble beaming material to, which doesn't make much sense. What ever it is has to be there since the 2280s...Or whenever Excelsior was commissioned.
Maybe there's a civilian facility so useful to Star Fleet they're willing to cater to it's size? ...I don't really like this solution.
The most reasonable thought I've had was the warp fields themselves where hard to make larger than 650 meters or so. Not sure if this really makes much sense either. But, then, what in warp physics really does?