well I never siad those were exact figures, it's not my campaign
I'm just saying, having a civilian juggernaught big enough to carry 6 million people or whatever you said would have to be huge. IMO the only way I could see it happen is if you have the GTVA military slaughtered and all hope lost, so you evacuate the best people throughout the worlds on one uber-evac ship.
This may be the case in your story, but I haven't really kept up with it, so yea.
EDIT: oh you're talking about Liners? Well how big are todays carnival cruise liners? I'd say compare it to the size of a carrier (carrier = GTVA Destroyer) and you got your length. Dunno if it's longer or shorter or not though.
Colony ships could be Jugg size, but 6 million is a lot of people.