@[email protected] If you want to upload a logo of maximum size then it is 978px * height. This will not take the full width, there is will 30px padding on either side. For logo image even if you upload image of bigger width of more than 978 it will take the width of 978px.
If you want to cover the full width of 1038px, try adding the header image from Appearance->Header but this will not have a link in it and will remain as a plain image.
.
.