I don’t often need to know the size of a folder but when I did I didn’t know how to find the size until recently.
When you use Finder’s List view to work with files on your Mac, the Size column tells you the size of each file, but when it comes to folders in the list, Finder just shows a couple of dashes instead.
Here’s how to view the size of a folder. Click File in the menu bar and hold the Option key, and Get Info will turn into Show Inspector. Unlike a Get Info panel, the Inspector panel is dynamically updated and will always display information for the active Finder window’s currently selected file or folder – including, of course, its size.