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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I don't think you can make accurate comparisons. I mean the sizes of Warthogs from Halo 3/Halo Reach to Halo 4 is pretty large. Although you could argue they are different models.