Rise cover photos

Hi all,

I know that this question is open in several discussions but I can't find any good answer. 

I really would like to know if there's any recommended size to upload cover photos in Rise (and all photos in general). All the answers I see is "the bigger, the better", that any size can work well, but the truth is that depending on the size of the image you have one result or another. 

Hope anyone knows the perfect size for images in Rise projects. 

Thanks :)