Different size video display for category page (smaller) & detail page (larger)

The best solution here is to use the built-in filter for embed parameters:

<?php
function mytheme_embed_defaults( $defaults ) {  
return array( 
    'width'  => 100,
    'height' => 100
    );
}
add_filter( 'embed_defaults', 'mytheme_embed_defaults' );
?>

This code can be added to your theme’s functions.php file and you can change the numbers to reflect the sizes that you desire. You can add conditionals as needed. Maybe something like:

<?php
function mytheme_embed_defaults( $defaults ) {  
    if ( is_category() ) {
        $defaults = array( 'width'  => 100, 'height' => 100 );
    }
    return $defaults;
}
add_filter( 'embed_defaults', 'mytheme_embed_defaults' );
?>

Would work best for you.