It should happen automatically, if Opus can see the change at all.
Does this happen even with a simple test collection? e.g. Create a new collection at the top-level (not nested), and add just one text file from your desktop to it, then edit the file. Does the size update after a second or two?
Under Preferences / Miscellaneous / Advanced, what are these set to? Is either set to a non-default value (it will be in bold if so)?
[ul][li]collection_change_delay[/li]
[li]no_external_change_notify[/li][/ul]
Changes to folders are not being detected has other suggestions which may be worth trying.
Do you have a lot of collections, nested collections, or collections containing tens of thousands of items, or anything like that?