The best you can have is a sparse disk image, you set the maximum size that the image can be but it only takes up as much space as the contents and a little bit extra.
I just made a 200GB sparse disk image and the overhead with nothing in it is 110MBs, not bad at all.
To do this, open up "disk utility" in "utilities". Select "new image" and in the "image format" section select "sparse disk image"