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I would think the GPSL files section would be a sensible place.
A very quick look with Google did not come up with an official limit on either individual or total megabyte limits, but in another groups.io I quickly found a 9 megabyte file. The total bytes used at that groups.io was already up to 125 megabytes.
I'll try uploading 25 and 50 megabyte test files to one of my groups.io file sections and see if that's accepted. Powerpoints can exceed that size with many included (excessively) hi-res photos, but that could be a start to knowing the limits for groups.io.
Paul, what is the largest powerpoint you have in the set?
I have the copies. I just need to find a place to post them.