Background image size issue

Hi, What is the recommended size for a background image for this theme? My client has a large image she wants to use— 2448px x 3264px @ 72ppi. It's almost 23Mb, so I know it's too larger to use as is for page load. It's more a texture than a photo, so both cropping and resizing are options. Can you give me a size to shoot for? Thank you