First of all you are using logo not header image and there is difference between those. Right size header image (1018px) would already cover the right side and the left side of your website. From there you would just have to add negative margin for top are to push image up like this
.header-image {
margin-top: -100px;
}
Margin can change from theme setup to setup. Just play around numbers to get the results you were looking for.
It is ridiculously difficult to make “full-size” logo because there are several other elements in header such as social icons. Personally I don’t recommend that route.