Is there an application (ideally for Mac) that would allow you to visualize / display graphically all the files and sub-directories within a directory?
If you don't mind working from the command line, the
tree command does what you want. It's more or less standard on Linux and comes with Windows. I gather it's not installed by default on the Mac, but it's easy to obtain.
Similar to @Josh's answer but for Windows, SequoiaView is a free tool that does a squarified treemap visualization of your directory structure.