Folder size calculation can be automatic or on-demand, depending on your preferences. Size calculation is performed in the background and does not block concurrent use of the lister, but it also causes disk access as the directory structures are examined so you may not want it to be done automatically, at least for certain drive types.
If you select one or more folders and run the GetSizes command then Opus will calculate their sizes and display the information the same way size is displayed for files. If you run the same command with no folders selected Opus will calculate the size of all folders.
GetSizes command: On the default toolbars, the GetSizes command can be accessed via Edit -> Calculate Folder Sizes (Ctrl+L).
Tip: Edit the GetSizes button and change its command to GetSizes NODESELECT if you don't want your folder selection to be cleared after the button is clicked.
You can also ask Opus to calculate folder sizes automatically every time you change folder. In Preferences / Folders / Folder Behaviour* the setting Calculate folder sizes automatically for... can be configured for all drives, all local drives, or fixed (non-removable) local drives, or you can switch it off entirely.
(*In Opus 9 and earlier, the Preferences page was called 'Options' rather than 'Folder Behaviour'.)
This general setting can be overridden for specific folders (as well as more specific types of folders) via the Folder Formats system. If you go to Preferences / Folders / Folder Formats, edit one of the formats and switch to the Options tab then you will see a setting called Get folder sizes. Normally this is set to default, which means the general setting is used, but you can also force it on or off for the particular format.
In addition to this, if you have added the Total File Count (AKA Files (total)) column or the Total Sub-Folder Count (AKA Folders (total)) column to your file list then Opus will always calculate folder sizes while calculating the total file/folder counts.
Cached folder sizes...
Some people like an add-on for Explorer called Folder Size for Windows Explorer because it caches the folder size information using a background service. This caching functionality is unlikely to be built into Opus as it requires a background service with full system access and memory overheads and there are issues with reliably tracking changes in the folder size data, especially while Opus is not running and during system boot and shut-down.
However, if you like Folder Size for Windows Explorer then you can use it inside of Opus. The add-on works just as well within Opus as it does within Explorer. In fact, it works better with Opus since, unlike with Explorer, it even works with Opus under Windows Vista. Ensure you have installed the 32-bit or 64-bit version which matches your version of Opus and then add its column to your Opus file display via Tools -> Folder Options and the Special category of columns (where all 3rd party shell extension columns are listed).