How do I control the output of a node rendered inside a specific region?
Inside Sidebar First, I can theme blocks by creating a block--sidebar-first.tpl.php template file. However, creating a node--sidebar-first.tpl.php does not work.
Unfortunately, Drupal doesn't have support to have node template based on region. You can check out this link.
If you want to achieve your goal you can use Display Suite module. Here are some screen-casts about how to control your content with Display Suite. Here's also a comment about this.