I want to have a banner image on header  - which takes entire width. what should be the size of image

 

Since that site is set to grow with the browser window, there is no specific or maximum value. You just need to pick size and then set the image to “width:100%". You need to balance file size and image quality. You could make the image 10,000 pixels wide, so it works on every screen, but then the image would get quite large. If you set it to 500, it might look very blurry on most desktops …