There are two possible things at play, the dimensions of the header itself and the dimensions of the background image. You can set both with css in style properties Header and Navigation > Header/Logo row
You can change the height of the header:
The padding there can also affect the position of the logo and dimensions of the header itself so some trial and error can be useful using both properties
Background image size is determined by the background-size css property
in extra.less EDIT: which can also do in the header/logo row CSS
Either specific dimension or various other such as
background-size:cover
(This stretches the image to fill the header
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