I think the main bed platform measures 54"w x 80" l. I am using one of these Exped mattresses (
https://www.exped.com/usa/en/product-category/mats/megamat-duo-10-lw-green) in ours which fits snugly side to side. Enough so that we usually have to do a little poking of the tent to get it to close right without pinching in the lid, but the comfort of the mattress is worth it.
So far, no insulation package for the tent (or otherwise) has been offered that I know of. The tent is very heavy material though for one. Lots of folks out there have done DIY insulation to the shell/structure if not the tent though. There is a German YouTuber who has some pretty extensive videos on insulating his CC for winter activities and I would say he has done by far the best, but even he hasn't added a layer to the inside of the tent from what I can tell.