Your theme is called “Metrolium”, and you can read more about it here:
If you wish to identify your website’s theme, use the Chrome Inspector (click CMD + C on mac), and click on the parts you see here:
Secondly, I’m having some trouble understanding what your problem is. You’re using a responsive theme, which means the header size will adjust according to which device you’re on. Is that the issue? What are the dimensions you’re getting? Is this the part you’re talking about?
Our website (https://forbrukslån.no) also utilizes a responsive theme, with our header changing its W/L size depending on the device.