Ok, so here's what we've got.
Compared directly to the Midway, the ship appears to be approximately 433 metres in length - we can't be certain of that, however, because while the screenshot was taken from a position equidistant (uh, that is a word, right?) to both ships, the camera angle is not entirely calibrated (basically, I took this shot from inside a ship, with the cockpit switched off - when I have a little more time, I'll do a few more precise comparison shots from a proper camera where I'll be sure there's no perspective distortion). Still, that should be fairly close.
The problem begins when the Pelican enters the equation. According to a list of ship lengths that you posted a few years back, the Pelican is 490 metres in length. This doesn't match up, scale-wise, with the Midway - if it were on the same scale, the Pelican would be 383.68 metres in length (...except that the Pelican is closer to the camera than the Midway and Nautilus models, so it's actually smaller than it appears to be).
In any case, assuming that the 490 metres length is correct, we have two scales operating here. If we assume that the Nautilus is on the same scale as the Pelican, we end up with a length of 553.5 metres.
The second comparison picture is limited to just the Nautilus and the Pelican. In this case, we can see far more precisely the difference between them, because the camera is much closer and equidistant to both ships. In this case - again, assuming the Pelican is 490 metres, we end up with a length of 570.8 metres for the Nautilus. This figure, I believe, is far more accurate than the 553.5 metres from the previous comparison.
Finally, if we assume that the 490 metres length of the Pelican is false, and go by the length derived from the first shot (383.68), we can, based on the second shot, estimate the Nautilus' length to be 445.76 metres (in this case, 1 pixel = 1.24 metres).