Using the following in your functions.php file should do the trick:
/**
* Configure default oEmbed video sizes
*/
function base_oembed_defaults($embed_size) {
if( is_single() ) {
$embed_size['width'] = 580;
$embed_size['height'] = 435;
}
return $embed_size;
}
add_filter('embed_defaults', 'base_oembed_defaults');
Add as many WordPress conditional statements as you like to match the desired aspect ratio for each template.