When a RM policy is applied in any document (PDF or Office), the original size of the doc is altered/changed? or how is handled/preserved this new "attribute"? or where is saved such information or properties? and if so, what is the specific size?
When a policy is applied to a document, some information is embedded in the PDF, specifically the URL of the Rights Management server used to protect the document.
I ran a quick test, the original PDF was Size on disk: 278 528 bytes (272 KB), after applying a policy the file was Size on disk: 282 624 bytes (276 KB).
Once a PDF has a policy applied to it, the security details will show in the Security Settings pane in Acrobat or Reader, see attached screen shot.
I don't know if there is anything available that specifically addresses your question (or my response), but here is a link to the LiveCycle documentation site...