Based on the suggestions of other contributors to this thread, I made my own document with mixed page sizes. It’s only six pages, three of each, and in non-sequential order so you can verify your machine chooses the right size for each sheet. Since I am unable to include attachments, I’ve put a link to a folder on my Google Drive, where you should find the document in both docx and pdf formats. I tested it out on my own printer (HP LaserJet 600 M601) using the PDF; both the test and the printer worked perfectly.
Here’s the link:
https://drive.google.com/drive/folders/1aGz_IFEknvjG-5eIYCzi-sLlLr5lCdBW?usp=sharing
Best of luck to all of you!