Apologies for the file_size question as I was trying it incorrectly. The correct way is to give only numbers (eg. %file_size% >= 10000). Its working fine.
But for the username, I am using below File Template code:
<?php $current_user = wp_get_current_user(); ?>
<!-- IF %file_added_by% == $current_user->ID -->
<tr> Code </tr>
<!-- ENDIF -->
Its not working. I have tried different combinations as well to include wp_get_current_user() function.