I’ve never heard of a file limitation for copying a single task in Project. Can you try something if you haven’t already? Open your project, save as a new name, delete enough tasks to reduce the size a certain percent, and see if you can copy/paste. If you still can’t, continue reducing the size until you can. I’m just trying to confirm that it is the file size that is creating the issue. If a reduced size fixes the problem then that’s the issue and I would consider that more a bug than a limitation (in my humble opinion).