Just a note, if you choose a set size, it will create an image that size, and you can add files up to that size. e.g. the screenshot above will create a 100MB disk image, it will be 100 MB on disk whether it contains 0KB or 100MB worth of files.
To create an image that expands to its limit, choose Sparse image from the image format option. e.g. A sparse image will be ~5MB if it contains 4MB of files, even if the size limit at creation is 1GB.