Well i found these pictures for the interior: http://colonialchrome.deviantart.com/art/The-Ghost-Exterior-503756450
I'm not sure how accurate they are, but It seems like the main interior space takes up the "core" area, and the sloped corner areas don't have any real interior space in them. I haven't even begun on the design, but i was thinking of making it in a modular way so that the various sections would snap into place with technic pieces, and then reinforced if needed, that way you could take the ship apart and still have access to the interior areas fairly easily without having to completely disassemble it all.
The images in the link above aren't to the same scale as what's written on this wiki page: http://starwars.wikia.com/wiki/Ghost
However, it seems fairly accurate compared to what i've seen from the show, plus it makes building it a bit easier, so i'll probably go for the smaller scale instead. That would make the bulk of the ship only 22 inches in length, since a lot of the extra length would be the front windscreen, and the engines at the back, which stick out quite a ways. The total length would be around 30.5 inches if you include the engines and front windscreen.
I've done a bit of shopping for some parts to make up the front windscreens and some tires to make the engines out of, as well as a boat load of white pieces for the exterior. I'm having a hard time finding parts to use for the cone areas on the engines between the larger and smaller diameters but was thinking of making them myself since i can easily machine something out of some kind of plastic to work if needed. I've also got the official Ghost on the way and some of the parts from that will go into the MOC, i should have plenty of free parts for everything else.