remove or update add_image_size

The add_image_size( $name, $width, $height, $crop ) function is graceful enough to handle multiple calls using the same $name. It simply overwrites the existing value:

$_wp_additional_image_sizes[$name] = array( 'width' => absint( $width ), 'height' => absint( $height ), 'crop' => (bool) $crop );

So that means that all you need to do to override the parent Theme’s definition of the custom image size is to ensure that your call to add_image_size() fires after the parent Theme’s call.

Assuming that the Parent Theme does like so:

function parent_theme_setup() {
    add_image_size( 'name', 500, 200, true );
    add_image_size( 'new-name', 400, 300, true );
}
add_action( 'after_setup_theme', 'parent_theme_setup' );

Then the parent Theme’s add_image_size() call fires at the after_setup_theme hook, at the default priority (i.e. 10).

(Note: if the calls are not wrapped in a callback, then they fire at the plugins_loaded hook, at priority 10.)

Here’s the critical part: the child Theme’s functions.php file is parsed before the parent Theme’s, so if you use the same hooks to fire your add_image_size() calls, they’ll get overridden by the parent Theme’s add_image_size() calls.

The solution is to use a later hook or a lower priority, to ensure that the child Theme’s add_image_size() call fires after the parent Theme’s.

This should do the trick, regardless of how the parent Theme fires its calls:

function child_theme_setup() {
    add_image_size( 'name', 400, 300, true );
}
add_action( 'after_setup_theme', 'child_theme_setup', 11 );

Note that we use the same after_setup_theme hook, but use a lower priority (i.e. 11 instead of 10. Callbacks fire in order from higher priority (lower number) to lower priority (higher number), starting with priority 0. So a callback hooked into priority 11 will fire after a callback hooked into priority 10.

Note also that the after_setup_theme hook itself fires after the plugins_loaded hook, so this callback will still override the parent Theme, even if the parent Theme is _doing_it_wrong() by not wrapping such calls in an appropriate callback, hooked into an appropriate hook.