Ratio of image size to size of files?

I am mussing about with a drive cloner that makes drive images. What is the general rule of thumb, if there is one, for the ratio of the size of the image to that of the disk it is backing up. e.g. my drive is 117Gb...so what size would the image be? Of that 117Gb, only about 30% of it actually has data on it...so the image size would obviously be smaller, or would it? I'd like to back up to a burned CD, but it asks for the size of the split files, the smallest being 650Mb. So, I don't know if the disk would be adequate or not. Obviously, more than one disk would be needed in the case of a larger image size...