Normally on Windows, only the size of files is easily visible - the (total) size of folders is not usually displayed. This is mainly because the file system does not keep track of the total size of a folder. Instead, displaying a size for a folder involves reading the contents of that folder (and all its sub-folders, recursively) and adding up the total size.
Opus can calculate the size of folders and display it in the file list just like it does for file sizes.
If you have Everything by voidtools installed, Opus can use its search index to perform this calculation significantly faster for indexed locations.
There are two ways you can trigger this behaviour in Directory Opus - manually and automatically.
To manually calculate the size of one or more folders, the simplest method is to select the folders in question, and then choose the Calculate Folder Sizes command from the Edit menu (or press Ctrl+K). If you execute this command with no folders selected, the size of all sub-folders in the current file display will be calculated.
Another way to trigger manual calculation of a single folder is to hover over it with the mouse until its tooltip appears. The default tooltip for folders contains the {foldersize} code, and so displaying a folder's tooltip triggers the counting of its contents. Using this method you have to keep the mouse over the folder (and thus keep the tooltip visible) until the counting is complete. This method may not work, however, if the info tip for folders has been modified to remove this code.
Opus can also be configured to automatically calculate the size of sub-folders whenever a folder (or certain folders) is read. The options on the Folders / Folder Sizes Preferences page control this.
You should look at folder sizes as a guide, rather than an exact size like you do for a file. Folder permissions can mean Opus isn't able to access some folders to count them. Junctions and softlinks can also cause confusion - Opus lets you control whether they're counted or not, but neither way is "right" or "wrong" and the calculated size can vary significantly at times.
Sometimes you may see a folder size shown with a tilde (~) character in front of it. This indicates the displayed size is only approximate. This can happen for two reasons:
Note than when using Everything to calculate folder sizes, you usually won't see a tilde, again for two reasons:
Because of this second reason you may get different results using Everything to calculate sizes versus Opus doing it the old-fashioned way.