@sportydave first you’ll need to adjust the header/logo size in your functions.php file. The code will look something similar to this –
//* Add support for custom header
add_theme_support( 'custom-header', array(
'width' => 360,
'height' => 70,
'header-selector' => '.site-title a',
'header-text' => false,
) );
It may be a bit different, but you’ll see the width and height for a custom header. Find out the exact size of your image and add it in there, then once you upload your image you won’t have to crop it.