Thanks Colin. I have already done the exact same steps, but the results are coming zero byte files and non-zero byte files.
1) General > Fields > Rule Builder and add Filesize [Map to column AU_Document_FileSize_Bytes]
2) Restarted the engine service and can able to select the drop down from the new category now as File Size.
3) Category > New and under Training > Field Text : File Size is Equal to 0
Note: I am using CP 5.8 version. It looks weird behavior. Anything missing here?
My understanding is that this Category is something dynamic query CP will create and execute against Database and return the results based on what we are adding. Is that any other option to enable trace or check behind the scene what is happening as I could't get much information from the Indexer logs.