Thanks, Vincent. 

Here is what I did according to the procedure:

• I located custom-document-formats.json
  https://github.com/xwiki/xwiki-platform/commit/82f8be6e0138bf763fab6c5a3434daebb5cb5876#diff-027df499b231874a89b7a66776c3cb591bed311b48822b08b39178376a2e390d
  which seems to have replaced document-formats.js mentioned in the documentation
• I copied its content in a local file with the same name custom-document-formats.json
• I edited it with Notepad and replaced the Presentation section with one from the documentation. I tried with both
  > "PRESENTATION": {"FilterName": "impress_html_Export", "FilterData": {"Quality": 90, "Width": 1024}},
  > and without the Quality setting and with several width sizes 1920, 2560 in separate test turns.
• Saved the modified local file.
• Added it in \xwiki-platform-distribution-flavor-jetty-hsqldb-14.10.8\webapps\xwiki\WEB-INF\classes
• Started the wiki server (Jetty 14.10.8 as the client's version, having an Office server with LibreOffice set on it and connected).
• Attached in different turns a PPT and a PPTX file to a wiki page and displayed it on the page using Office Viewer macro.

Result: it did not change the viewer size in either full screen mode or outside the full screen mode.
Extra note: I tested with both local LibreOffice 7.4.1.2 and 7.5.8.