An alternative would be to create a watermark with a stepped approach.
Create a watermark with different instances which all have a different font size, and make the placement dependant from the page size.
This allows a simple workflow
The only option I see is two workflows one for each sheet size. The advanced prepress module does not have "if/else" capabilities.
The Watermark width field does not accept FFCore variables. Dimension fields which have a "link" icon can use FFCore variables (see screensnap). If you have the Advanced Automation license, the Route node can be used to accomplish what you are looking to do (see screensnap).
i've searched the forums also, but am looking to create a workflow to create low res pdf PROOFs with some document size information.
My graphics can be coming in as 24"w or 120"w and i want to have the width of my watermark adjust based on the document size.
I also want to accomplish the same thing for a PDF watermark.. it seems the width field doesn't accept the $FFwfDoc.trimWidth$ variable