a bit of threadomancy : i am currently reading the gordon rennie books, and the length quoted are 3Km for a Cruiser and 30m or 100m for a torpedo. I think this is due to a mistake in English measure conversion somewhere. Fury interceptor are givens as weighting 200 tons, and Starhawk Bomber 300 tons.
I think the "skyscrapper sized torpedo" was talking about a ground based defense torpedo. I think that kind of comparison was used in the Ultramarine book where they fire such a torpedo at a Hive ship, and something equaly huge for the torpedo shot against the Iron Warrior and Legio Mortis in "Storm of Iron". But those are supposed to be bigger, and possibly able to take down a battleship or a hive ship in one hit.
And as for the illustration, when the artists get carried away anyway. I think everyone remember this illustration with a warlord titan towering among mountains, for example...
wait, here you go :
The servicing door you can see under the gun barrels would suggest that the height of this weapon would go in the 30m range (if it's 2m high, as it can fit about 15 times), while this is supposed to be the height of the entire Titan...
