I have about 30 GB occupied on an 80 GB C drive. Macrium images of that drive are typically about 10 to 12 GB in size, with the default compression level. I used EaseUS imaging for a while and as I recall it produced files of about the same size.
I'm not sure what the built-in Windows tools uses.
Many people keep multiple images. I normally have 3 of various ages totaling around 35 GB, so you might want to allow for more than a single image. I'd just keep them on another drive---no need for a separate "images" partition or anything like that.