The accepted landscape photo proportion is 16:9 = 1.77. Recently, I have been experimenting and arrived at 14":8"= 1.75. for paintings. This is close enough.

But I add another 1/2 " to the width (so 14" x 8.5") for some allowance either for the foreground or the sky. I think this allows for a more panoramic landscape.

I have shared 2 painting in "Images Sharing" done in this size. Let me know your thoughts.