Hi and welcome to the guld! This seems to be a nice start.

To save you picture, I found that the best way is to use Jpeg rather than png unless you have transparent pixels. Lowering the quality of the Jpeg when saving is usually better than reducing the dimension of the map. I still don't know the limit. It's 4,67 megabytes in size but for the dimensions I think it's 5000x5000 or more.