The Ares class is also significantly smaller than a Constitution class, I think. In one shot, you can see a guy in a spacesuit standing on the hull, which gives us a pretty good scale of the ship (and confirming my suspicions that the ship has fewer than 10 decks)
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As to size comparison I found this but I don't know how accurate it is.
http://excalibur2008.deviantart.com/art/Axanar-Ship-Chart-from-The-Light-Works-465842449