I just learned the hard way that Apple Photos is NOT iPhotos. After spending many hours rearranging and organizing old family photos I find out that the Slideshow export
1. doesn't use the order I have
2. doesn't let me specify the size of the JPEGs
What's more, later versions of iPhoto also changed/got rid of features I used in iPhoto 6, making them equally useless for exporting a slideshow.
Fortunately, I have an old Mac G4 tower running iPhoto 6 so I'll be able to do the job. Unfortunately, I have many hours of work ahead.