This page lets you control whether or not Opus automatically calculates folder sizes when you navigate to a new location. If automatic calculation is disabled you can always use the Calculate Folder Sizes command in the Edit menu to trigger a manual calculation.
The list at the bottom lets you override size calculation on per-folder basis. You can add specific folders or use a wildcard string.
For each folder you add to the list you can use the options at the bottom to either enable or disable folder size calculation. Note that this setting applies within the specified folder. For example, if you add C:\Temp to the list, and set it to Calculate size, then navigating to C:\Temp would trigger a size calculation for any sub-folders within it. It doesn't mean that Temp would have its size calculated if you navigated to C:\.