I found something that's really cool:
http://halo.bungie.org/misc/sloftus_scalecomparison/1280.html
This chart has scale comparisons to objects in Halo. It looks like it took a lot of work.
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So -blam!-ing cool. I have an obsession with gargantuan structures, and the Ark is just about as -blam!-ing enormous as it gets. How big is Requiem? The Didact's Metapod-esque ship? The Cryptum?