Hi Alex!
I faced the same problem you did, but managed to FINALLY figure it out. You should go to header.php, not custom-header.php. Once you’re there, look for this:
<?php if ( get_header_image() ) : ?>
Two lines below that should be a width="< cooooode; ?>" height="< coooode; ?>" alt="" />
Delete the whole of the < cooooode; ?> for both the width and height and replace them with your desired size. So it should look something like this instead:
width="450px" height="450px" alt=""
This was what worked for me — hopefully it’ll work for you as well!
