There's a lot of different estimates on the total size of the background images here, and a lot of them don't even elaborate on most of the assumptions they are making. Fortunately a few of the posters have already explained this as well, but everything depends on the amount of pixels you are expecting that the game backgrounds have in total, and the compression that can be applied to those images.
From this thread and a few other sources, it seems to me that lossless compression could get the file sizes down to about 4 bytes per pixel. Adding some lossy compression as well might maybe get the sizes down to even 2 bytes per pixel, but that is contestable.
Another question is the total amount of visible "screenfulls" of background images we are talking about. I would offer a ballpark figure of about 100 maps made up from 6 by 6 screens, so a total of 3600 about screenfulls of background images. That is quite a lot, but I am factoring in some larger exploration areas as well, which might not be as time-consuming production-wise as detailed city areas or complex indoors locations, but will nevertheless take up space on the disk.
So, using a native resolution of 2560x1440 pixels, we get a total background data of about 13 gigapixels. Using a 4 bpp compression, that translates to about 50 GB of image data, and using a more optimistic 2 bpp compression, about 25 GB of image data.
For a game that is likely released in 2014, I don't think that size (plus all the rest of the game data) should be a problem for anybody. The kickstarter was for not only a PST/BG/IWD style game, but also a good-looking game, and you can't expect a game that has good-looking (by modern standards) game with 2D-environments to be less than a DVD in size anyway.