For now, I'm using this technique posted by wgroleau on https://apple.stackexchange.com/questions/402327/how-do-i-filter-videos-by-size-to-identify-the-largest-in-photos-app-on-osx
Here's another way:
plug in an external disk or storage.
Create a folder on it.
Open Photos and select all the videos.
Drag them to the new folder.
Wait a LONG time for the export to complete.
Sort the new folder by size.
OR, if you're happy with keeping them there, go back to Photos and delete them. That will save more space than is in the new folder, because Photos creates a lot of supporting files for its contents. (Actually the space is not reclaimed until you also clear out the "recently deleted" collection.)
If you don't get a notification when the export completes, you can compare the number of items selected in Photos to the number of items in the new folder.
Mcfugu: You can monitor the status of the transfer by looking at the circle at the top left of the Photos window that shows the status of task completion. Clicking it gives further details.