Well, I'm fairly confident that a Negh'Var would have considerably superior endurance to a D'Deridex, despite the size difference. The First Battle of Chin'Toka gives us an excellent direct comparison of Klingon, Federation and Romulan armor performance against the same weaponry at the same time, and the Klingon armor comes out the superior of the three by far, while the Romulans' is notably inferior. In the very beginning of the battle when the weapons platforms come online, we see a D'deridex get hit by two beams on its 'forehead', which cause major damage and a rapidly expanding hole. Moments later we see the USS Galaxy get hit by three such beams on its belly with a similar but much smaller hole (easily 2-3 times smaller), with notably slower expanding after effects. We later see the same weapons inflict much smaller holes on Klingon warships, even the BoPs, with expanding after-effects moving notably slower than they did on Federation ships. We even see a Vor'cha at one point take six of the torpedo/pulse weapons to a nacelle, enough to destroy any of the Federation cruisers and inflict significant damage to even the USS Galaxy, yet the damage is relatively minor (the nacelle starts visibly venting plasma, but no gaping hole is visible, and the hole we see blown into the USS Galaxy's saucer by just three such shots should have been just barely visible at that range, and six shots should have definitely been visible, if the armor performance was comparable to Federation armor).
The direct comparison of the D'Deridex and Galaxy performance indicates that the Galaxy's armor endurance is easily 3-4.5 times greater than that of the D'Deridex (a hole 33%-50% the size of that on the D'Deridex inflicted by 50% more firepower), and quite possibly even more than that. We also clearly see that Klingon armor demonstrates performance that is at least twice that of the Federation (though probably no more than three times - the nacelle on that Vor'cha was quite visibly venting plasma). This would easily put Klingon armor endurance at six to thirteen and a half times that of Romulan armor endurance, significantly surpassing the size difference between the D'Deridex and Negh'Var even on the low end.
Coupled with the known technological inferiority that the Romulans had to overcome (an unlikely occurrence, given the rate of Federation technological advancement and the Federation-Klingon alliance), the fact that Romulans do not use M/AM reactors but an artificial singularity power source, which could well require considerably more space for comparable energy production, and the fact that they also apparently do not use conventional impulse engines, either (the D'Deridexi have no sublight engine exhaust ports; given the general strategies and tactics of the RSE, and the fact that we see other Romulan ships that do have impulse exhaust ports, I would speculate that the D'Deridex uses an alternative method that is more stealthy but that consumes more space), I would say that, despite the massive size of the D'Deridex, she is ultimately a notably inferior direct-combat vessel to the Negh'Var. I would honestly expect the Negh'Var to be able to compete about equally with two D'Deridex in a direct combat engagement.
I will note, however, that the D'Deridex is most likely NOT designed for that kind of engagement, at least not as her primary combat role. The D'Deridex is most definitely a potent and powerful battleship with 360-degree arc coverage with large and powerful weapons, but this is most likely ancillary to her primary combat role as an ambush predator. She is designed to be able to fill the role, but it's not the primary role she's designed to fill. The D'Deridex's main 'mouth' gun is probably the single largest disruptor cannon ever built in all of known Trek (barring hyper-advanced ancient powers), or at least fielded on an actual starship and not a dedicated battlestation. It has also demonstrated more firepower than any other conventional energy weapon ever observed (capable of dropping the shields of even a Galaxy class starship down to 30% with a single salvo of seven pulses). Now, we know that the D'Deridex is only roughly comparable to the Galaxy, as implied throughout TNG, meaning that in a sustained fight the overall firepower of the D'Deridex is only roughly comparable or possibly a little superior to the Galaxy's (we know that spaceframe endurance is roughly comparable or a little inferior to the Galaxy's based on armor performance seen in DS9, and ship volume), so the D'Deridex obviously cannot sustain that kind of firepower, but in an opening salvo the D'Deridex can probably throw out more firepower than any other ship currently fielded by the major powers on this side of the galaxy. Now, she's something of a glass cannon, but a stealthy glass cannon meant to devastate or obliterate the enemy with its opening salvo as it drops its cloak fits precisely with the standard Romulan Modus Operandi.
So, in a straight-up fight, I would say that a single Negh'Var would cream a single D'Deridex, and probably beat two D'Deridexi, though it would sustain heavy damage in the process. A single D'Deridex ambushing a Negh'Var would probably be able to inflict some damage, but would also probably still lose. Two D'Deridexi ambushing a Negh'Var would probably win, though they would likely sustain significant damage and maybe even lose a warbird.
