I'd like to think the new one is ~ 900 meters.

And the TOS and TMP versions as well.
I've always thought the original sizes for the ships were too small. Seriously, how could 430 people live *and* work on a ship 280m long, 111m wide, in the configuration of the Constitution class? If it was all one piece, I'd say yeah, entirely possible, but with a saucer-can setup there's nowhere for supplies, spare parts, machinery, work space, guests... You see where I'm going with this. 900+m works *much* better.
Exactly, I completely and totally agree with you.
It's not just workstations and labs and private quarters. Even if you bunk them all up, they still need room for facilities. They need quite a lot of room for consumables, or at least base matter for the resequencers. There needs to be room for water. Every part of machinery should have at least one spare, preferably more. There needs to be room for evacuees (thus not all quarters should be taken all the time). There needs to be room for distinguished guests. Room for shuttles. And room to service those shuttles. Room to armories and torpedo bays. All sorts of stuff.
Remember, it's an exploratory ship. It's gotta do without a starbase for quite a long time, in the middle of no man's land. It should be self-sufficient for at least how long it's mission is supposed to last.
Now, if we'd all have shrinked ourselves in the future to be 50cm tall on the other hand, the old length might just work...
The main thing is they don't have replicators yet. Or at least, we assume they don't. Because, when you have power to spare, you can create everything at exactly the right moment you need it. That should definately save a lot of space in the future. So actually, the 24th century vessels should be
smaller, not larger.