With the recent major update from Ghost to version 3.0, the design of the Casper theme was also improved. This also slightly changed the size of the publication cover.
Even before that it was possible to select basicly any image as a cover. Depending on the aspect ratio of the picture, more or less of it was actually displayed.
To create the perfect cover, you want to know which aspect ratio the optimal cover must have. Since the cover adapts dynamically to the width of the screen, there is no 100% answer. However, it can be assumed that most monitors have a 16:9 format.
For this format, the optimal size of the publication cover is 1900x400 px, which corresponds to a format of 4.75:1.