Right, I'm not the world's best at image hosting, so I really hope this works...
I always really liked the comparative size charts for Star Wars vessels in the old West End Games Dark Empire books, so I thought I'd have a go at putting some together for Dark Heresy and Rogue Trader. These are "best guess" sizes for a handful of Imperial vessels.
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The sizes for the lunar class vessel, the Pale Sepulchre and the Bountiful Beast are correct, based upon information in Rogue Trader and the Radical's Handbook.
The size of the Emperor class battleship and the Miserichord are guesses on my part, based in part upon descriptions of the vast scale of the MIserichord in DH, and the side by side estimation of the different scale of the BFG miniatures. But I think the scale of these vessels feels right.