It's pretty easy to do. You can see how big it is in relation to Saturn's rings during the arrival cinematic for the new Crucible map, The Dungeons.
Saturn's rings are 73,000 km wide. From the cinematic, the Dreadnaught is about 0.07 times as long, and in the same plane as the rings. That makes the Dreadnaught...
[b]5110km long![/b]
That's over 5 million metres.
From other, better quality images of the Dreadnaught, I've calculated that one side of one of its square ends is about 0.3 times as long as it's length. Therefore, that gives a total volume of...
[b]1.2 x 10^10 (12000000000) cubic kilometres![/b]
So, yeah. It's pretty damn big.
[b]Edit:[/b]
Obviously, I am not expecting that we will be able to visit even a tiny fraction of this. Just like how we cannot visit all of the Earth, Venus, etc.
Also, I find it likely that Bungie does not actually intend for it to be this big, and instead either didn't realise (or expected other people not to realise) just how big Saturn's rings are. That, or it's just to make it look more badass.
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I did some measuring with a different reference point, which I think removes the depth perception question completely. I used this trailer [url]https://www.youtube.com/watch?v=Jm8cxcmVA8Y[/url]. I'm pretty sure that space rock that hits the ship is Comet 67p/Cheerio-Emelianenko(if it's not, then none of this applies). It's listed to be about 2.5 by 2.7mi. Just before it explodes on impact with the dreadlock, I measured it at 2/16in or 1/8in or 0.125in. Then I measured 3 full screens of the ship passing by, from around 0:40 to 0:55, with each screen being 19.75in, for 59.25in(3 x 19.75). This is where my final number could go off course, because I just guesstimated that the final back section is a little less than a third of the whole ship, and called it 17in(close to 1/3 of 59.25), for a total of 76.25in So: 1(inch) divided by 0.125 = 8 "rocks" per inch... 8 x 76.25 = 610 "rocks", each rock being we'll say 2.6mi across so 610 x 2.6 = 1585 miles or 2550 km If they crafted that rock to be this specific comet, I feel like they would be aware of its size. You never know though, it coulda just been an artistic choice. I reserve the right to be completely wrong or mistaken (I'm no mathemagician) and maintain my dignity tho. It seems like no matter how you look at it, Oryx has an enormous dreadnaught.