The "Contemporary" theme for your agent website uses something called "responsive design". That means that it resizes and changes the content arrangement so it looks good no matter what size screen you view it on. The best size for this background is 2200 pixels wide by 1182 pixels high (2200 x 1182).
That ratio works out to 7:4 so as long as your image matches that ratio, you can use one smaller than 2200 x 1182. Example: if you have an image that is 1600 pixels wide, your height needs to be 860 pixels.
Please note: the theme will automatically enlarge smaller background images so they take up the full space thus making them look lower quality.