The image size can be whatever size you need, we just have it set at the 300px/60px width/height by default (as a start).
To upload a logo, go to Appearance > Customize > Header Image and you can upload a logo from there.
It’s recommended to upload the exact size that is specified in the functions file so you don’t have to Crop your image in WordPress.
If you need to change the default Image Size, look in your functions.php file and change the code that looks like this –
//* Add support for custom header
add_theme_support( 'custom-header', array(
'flex-height' => true,
'width' => 300,
'height' => 60,
'header-selector' => '.site-title a',
'header-text' => false,
) );
If you can, first try and upload your Logo/Image on your Media Page at it’s original size, so I can take a look at it to see if it shows up blurry on my end (before we upload it to the Header area).
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