> But. What size is too large? If I know what to watch for, I can avoid it.
I don't know, depends on your server. I am sure I already posted this link on some other topic: https://www.zenphoto.org/news/problems-with-albums-and-images/
the more images the more server load. And if then metadata is inviolved that is even corrupted (the f file is read to get metadata) that also may add up.