[quote=Sundered Echo]
Matt you know that you can fit a company of marines and all their toys on a (much smaller) Strike Cruiser with room to spare. Chapters operated out of cruisers during the Heresy Era with no trouble at all.Also, marines don't take up all that much space. Think about this: we can, right now, fit 5,680 people on a 332.8 meter long carrier (Nimitz Class) designed to hold a huge number of aircraft which, while nowhere near the size of a thunderhawk, still take up a considerable amount of space. While a marine ina rmour would most likely find the corridors in such a ship impossible to navigate, the scale should still make a point on its own. Even if you doubled the size of the rooms to account for it, you'd still get a good 2000 people on it.Incidentally, Thunderhawks are barely pushing thirty meters. Source (Taken from a Forge World Imperial Armour Book, which is only one step removed from a Codex in terms of Canon.)I'm pretty sure you could comfortably downsize to a 2km cruiser and keep all the toys, and if its a heresy era ship, as many are, it may very well have a teleporter. Also, the mechanicus can still build a lot of their lost tech, they just cant improve it as they don't understand it, as well as generally taking a very long time to build it.
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We've already discussed this, it's not Heresy Era, I've said this a number of Times. Also, as you said before, size, in the Warhammer Universe is debatable, as things are constantly being changed. Lexicanum (My go to source for Warhammer knowledge) said that [url=http://wh40k.lexicanum.com/wiki/Turbolaser_Destructor#.U4xWbPldVv8]Thunderhawks can be outfitted with Turbolaser-Destructor's[/url] (the same cannons mounted on a [url=http://img4.wikia.nocookie.net/__cb20110720111608/spacemarine/images/b/b6/Reaver_titan.jpg]Reaver-class Battle Titan[/url]) If so, then a Thunderhawk would have to be pretty fucking huge to carry a generator capable of powering it up a weapon of that magnitude, without losing it's flight capability.

And as for the ship... Well, after looking into it, apparently things have got pretty fucked up as far as size goes, apparently, now, the smallest Capital ship that the Imperium fields is the Cobra-class Escort Frigate at 1.5KM. unfortunately, that thing has little to no space for an assault force, built like Defiant from DS9. Plus, of course, dat look... Vengeance is an awesome shape for a ship.

Fuck it, Let's say that the Vengeance is 2.5KM and have done with it, Who's gonna complain? Matt Ward? He can blow me.