289 meters. 442. Doesn't matter really. Both the TOS and SNW versions are the same ship just a decade or less apart.
As noted, too, it isn't just the meters but the mass. As I noted decades ago, at 190,000 tonnes the TOS Enterprise at 289 meters would be 90% as dense as water. That is, she would float. The Discoprise has vastly more volume. I haven't obtained it from a model, but if we estimate by boosting the TOS Enterprise to 442 meters then the volume would be about 755700 cubic meters. That puts us at about a
quarter of the density of water and twice the density of air.
(Traditionally nacelles have been viewed as particularly dense, which could give the Discoprise a fighting chance at floating in mid-air like a hot air balloon without them.
Oh, and before someone tries to argue 'dry' versus 'wet' mass, do recall (a) the fuel is deuterium, (b) the crew is just 200, and (c) we have the Kobayashi Maru stats which show even a cargo ship to have a dead weight tonnage less than 190,000.)
To recap, using the idea it is the same ship:
2254 ("The Cage")
Look: TOS Enterprise (shots reused in TOS)
Model: 33 inch and 11 foot pilot config
Length: Meh ... I haven't scaled the bridge view with its angle issues nor found one in a quick search, but by eyeball we'll say in the neighborhood of 300–ish meters.
Mass: Not stated
Late 2250s / Early 2260s (Discovery, SNW)
Look: Discoprise, Discoprise Bewindowed
Model: CG variations
Length: Varies, currently 442 meters
(Has been shown at 515 alongside 750m Discovery, displayed at 289 and 442, latter seems to be settle point)
Mass: 190,000 tonnes
Mid-to-late 2260s (TOS)
Look: TOS Enterprise
Model: 33 inch, 11 foot, et al., various configurations ... oh and CG, too.
Length: ~289 meters displayed on screen
Mass: "
almost a million" tonnes
Early 2270s (TMP)
Look: TMP Refit Enterprise (later Enterprise-A)
Model: 8 foot TMP model, various partials, AMT/ERTL
Length: 305 meters
Mass: Not stated
(Note that all lengths above are contested via comparison with interior views, though the SNW ship has the largest discrepancy by far thanks to the turbolift caverns.)
Now, there are various possible takes on the data, again operating under the "same ship" hypothesis:
Some think the current 442 meters for the Discoprise overwrites not only the previous 289 and 515 for the Discoprise but also the 289 meters for the TOS Enterprise, whether by administrative fiat of retcon (though no such statement has been made) or by logical necessity of refitting or both, with no apparent thought given to the mass question. By this argument all the entries in the list above should be 442 or so, and presumably we're also looking at a flat rejection of the canonical TOS mass.
Some think 289 is the standard yardstick for starships throughout the original productions and 442 is an aberration, with
Ronald Held hypothesizing at one point that the Discoprise was a wartime build that replaced and is later replaced by the TOS style ship.
Various other possibilities exist, but I would like to point out that Star Trek: Picard provides an easy solution to this conundrum.
Recall that the Luna Class Titan, a post-TNG-styled ship featuring an oval saucer, upper pontoonish base for an upper Miranda-esque pod thing, neckless round-bottom secondary hull, and underslung nacelles was explicitly "refit" into the Titan-A, a ship with a TMP-style saucer in the front 2/3rds, large jagged impulse stuff behind that, an old traditional neck leading to a boxy secondary hull, and upswept nacelles.
That is all to say, the argument that the Cage and TOS Enterprise must also be 442 meters out of some antiquated notion of engineering rationale or logical consistency has been surpassed by the new generation of Star Trek shows. Like they said of the TNG era in 1987, aesthetics has surpassed technology and we can build -- and rebuild -- machines in which man would be proud to fly.
This Titan-esque refit hypothesis neatly avoids claims of contradiction and all the messy arguments that come of it, prevents folks from having to pretend they can't read the "Enterprise Incident" screen even in HD, keeps us from having to pretend Scotty doesn't know his ship's mass, and so on. It solves all the problems of the same ship hypothesis.
So, in 2245ish the Enterprise is built at the San Francisco fleet yards (in orbit ... sorry, JJ), then sometime between 2254 and 2257 undergoes extensive refit that lightened and lengthened the ship tremendously. Maybe the old parts were even retained. We might prefer that this refit be right around 2257 since the ship suffered a massive multi-system failure requiring a tow to dock and lengthy repair. In 2258 the ship was restored, then it saw another refit apparently featuring installing the secondary hull pendant slider, cutting windows into the outer saucer framing, and other changes, all of which were complete by 2259. At that point the ship embarked on a series of missions and was also a testbed for new technologies including a holodeck and a follicular antigravity device in the enlarged Captain's quarters, presumably replacing the antique television he had in 2254.
Sometime before 2265 the ship is refit once again, largely returning to the 2254 configuration (hopefully with those retained parts). This refit shortens the ship tremendously yet also increases her mass by a factor of four or five. After a successful five year mission, the ship is refit yet again circa 2271 into a new design of roughly the same size as she'd just been.
Now, with a non-contradictory hypothesis on the table, is there any good reason not to accept it besides foot-stomping my-way-or-the-highway behavior? The way I see it, even the old-time TOS and TMP rescalers who were in the low-300 meter range ought to like this basic framing even if they hold 289/305 in disdain, and they still get to fight 289/305 without impacting or insulting the Discoprise 442 crowd.
Everyone is happy, right?