Resize a disk image
You can read and write to a disk image or change its size. For example, you can make it smaller to save disk space or make it larger to store more information on it. To resize a disk image, first close the image by ejecting it, so it doesn’t appear in the sidebar when you open Disk Utility.
Choose Images > Resize, select the disk image file you want to resize, then click Open.
Type a new size, then click Resize.