Just add the following code and edit the dimensions as desired:

1
2
3
4
5
6

function mycustom_embed_defaults($embed_size){
	$embed_size['width'] = 690; // Adjust values to your needs
	$embed_size['height'] = 420; 
	return $embed_size; // Return new size 
}
add_filter('embed_defaults', 'mycustom_embed_defaults');