A screen resolution of 360x640 Pixels is the best for all users worldwide, regardless of their device. Anything above the state size would make it challenging for users to properly view the images sent through Email.
We all know that users have access to smartphones more compared to other devices. It is why 360x640 Pixels is the ideal size on a mobile device, as it helps a person read the contents of the image instantly, rather than having to change the mobile view settings.