The way this works is that it first creates a Letter-sized PDF using the regular Generate PDF Document action. This gets fed into a Python script that scales all the pages and creates a new PDF.
You'll need to do a little bit of math if you want to modify the margins because of the scaling. Everything gets scaled up by a factor of 1.18 (13"/11"), so you need to take that into account when setting font sizes and margins in the Generate PDF action. You also need to add 1.3" to the right margin – this way, the printed area of the Letter-sized PDF has the same aspect ratio as the end result. I hope that makes sense. The workflow is currently set up to have a margin of 0.59" (0.5 * 1.18) on all sides.