After extensive experimentation, it seems that the preview display limit is 2MB for me, here and now. Bigger than that and they don’t display using Embed Any Document (though I understand that it’s not your issue, but Google’s). That means that my 30MB PPT presentation (already maximally compressed) must be split into 15 pieces, which is not workable. So I saved it out as jpegs and uploaded all those to a webpage instead.
Question: Is using Microsoft better than Google for this purpose? I.e., does it accommodate larger files in the preview display mode?