Creating a relation within drawing that uses a drawing parameter

I think the answer may depend on how you are creating the PDFs.

If this is a manual process (directly from Creo), then Martin's approach would probably make sense.  You could also create a series of mapkey to automatically create the note, export to PDF, and then delete (or hide) the note.  In this case you would make one mapkey for each sheet size and then just call the correct one.  (pdfa, pdfb, pdfc, etc.)

If PDF creation is an automatic process (like additional files generated during publishing by Windchill), then I would recommend you consider something like Fishbowl's LinkAccess solution.  We use this to watermark PDF's during publishing with the lifecycle state, approver's name, and approval date.  The software does allow you to create separate recipe files by sheet sizes, state, etc.