Cover images have a fixed ratio of 1:1, i.e. ideal images are square.
We recommend making your square images 1080px x 1080px. This way, the cover image will be of high quality.
Other common dimensions that might fit are 1000px by 1000px or 640px by 640px. You can always choose an image in a different dimension, but you'll need to crop it first to fit the square ratio of the cover image.