The size of your banner image depends on your specific site template (see here for more on templates). Here are some general guidelines to follow:
For a split-width template, the actual dimensions vary, but they are generally between 640x304 and 680x334. Please see the image on your homepage or contact us for exact dimensions.
For a wide template, the image should be 1004x336.
For a full-width template, your image should be 2500x785. To ensure faces or important parts of the image don't get cut off on larger screens, they should be in the middle-center of the image.
For all Standalone Donation and Ticketing Pages, the main image should be 618x270.