Do you mean that “lunch” image below the header in the default theme? I think this might be called a “hero” image in other themes. It’s size is 1920×1281. The other images that come in that same folder are all 2000px wide and 600 to 800px in height.
The ‘header image’ can usually refer to two distinct elements:
– the classic header image (which is core WordPress functionality); its size depends on the site width and the header height (both configurable theme options);
– the landing page’s static slider image (which is a theme feature); the image and its size (height) is directly configurable in the theme’s landing page options.