Well there is a Ship in the last book (which is not in the compiled volume) called "The Empty Chair" called the Tyrava. I was built by the "Ship-Clans" in (in secret) the outter territories of the Imperium and was described as: "It was a triple-hulled design, all three of the huge backswept, cylidrical hulls mounted parallel, at hundred-and-twenty degree angles, in a mighty framework. " The ship was also mentioned to have no visible, external warp nacelles. Now, the vessel was also described as being over 3 km in length (I personally do not buy that, as in the series ship sizes were wildly attributed, especially for the era of the book's timeframe).
Anyway, I drew up what I precieved the Tyrava, might have looked like, and scaled it way down from 3 km (although I'm pretty sure I will rescale it again so that its just a bit smaller. Again, I cannot see the size of certain vessels being larger than a certain scale).