Adjusting header block to contentSize or wideSize

Hi there.
I started making my new website with BlockBase. I’m stuck on setting the width of the header in templates.

The header block doesn’t have a setting to change width to ContentSize or WideSize. The header is full width. The only way I found to change the width was by heading the header block itself, which opens a new windows with the header in the center, and I can slide the edge of the block to adjust width manually. And there is no value shown, therefore no way of knowing if the width is precisely what I want.

Adjusting it with .site-header in style.css doesn’t work either.

So, how can I set the header to the same width as the content ?