There is no exact size we can recommend as we will resize any image as is needed (we need to resize it depending on what screen the book is viewed on) but a rough guideline for a great background image is about 2000px wide. This should give decent enough quality in most situations.
Again, the same applies for logos, we handle the resizing to fit the layout. But you want the logo to look sharp and professional, so generally speaking bigger is better. Just upload the highest res, best quality logo you have available for the client. Generally, anything less than about 500px wide is not going to look great in certain situations