how to search by file size? What to type in the searchbar for a specific file size?
The easiest way is via Tools > Find Files, which gives you an easy to use UI that includes a size option.
The search field at the top-right of the window uses Windows Search, which has its own syntax. Windows Search in the Opus manual has a link to Advanced Query Syntax on Microsoft's site. I don't know if there is a newer/better document than that. Microsoft haven't done a good job of documenting Windows Search. But that page gives examples like
size:>500 to find files larger than 500 bytes.
I would use Opus's Find Files UI instead of Windows Search, as it's a lot more straightforward.