查看函数wp_embed_defaults() 在里面wp-includes/media.php:

function wp_embed_defaults() {
    if ( ! empty( $GLOBALS[\'content_width\'] ) )
        $width = (int) $GLOBALS[\'content_width\'];

    if ( empty( $width ) )
        $width = 500;

    $height = min( ceil( $width * 1.5 ), 1000 );

    return apply_filters( \'embed_defaults\', compact( \'width\', \'height\' ) );
}

add_filter( \'embed_defaults\', \'wpse_76102_new_embed_size\' );

function wpse_76102_new_embed_size()
{
    // adjust these pixel values to your needs
    return array( \'width\' => 1000, \'height\' => 600 );
}