The default header image size is 1920px x 521px

The default logo size is 41px x 41px

@Darryl, the header image won't display as 100% in browsers. How to rectify that? And I want a logo size of 150px width x 41px height. What I will do to change this?

Paste these in the custom css section in you admin panel for the Simplicity theme. Not sure what you mean by 'It won't display 100% in browsers"

Try this for your header
.ow_header_pic {
    background-size: 100% 100%;
}

You can try this for you logo

.ow_logo {

    background-size: 100% 41px;

   width: 150px

}

If this is the same as the post you had in the theme forum section, I gave you this for that site.

.ow_logo {
    width: 140px;

}

From looking at the education site you have listed in the Forum in the theme section; I am wondering if that header image has white edges that need to be cropped around the image. I think it is displaying at 100% width, but if there is blank space on the left, and right of the image, that is why the visual part won't stretch across the screen.