functions.php

/*modify youtube dimension*/
function mycustom_embed_defaults($embed_size){
    $embed_size['width'] = 645; // Adjust values to your needs
    $embed_size['height'] = 450; 
    return $embed_size; // Return new size 
}
add_filter('embed_defaults', 'mycustom_embed_defaults');

or

add_filter( 'embed_defaults', 'modify_embed_defaults' );
function modify_embed_defaults() {
    return array(
        'width'  => 750, 
        'height' => 375
    );
}