Some window managers allow you to lock a window's size. I don't think Tails' does. Tor Browser doesn't (yet?) provide any way to lock the window's size or reset it to default. The only way to correct it is to restart Tor Browser.
As a quick and dirty solution, hold the Alt key and click anywhere inside the window (not the title bar) and drag to move it. In some window managers, it's the Windows key, so try that if Alt doesn't work, otherwise consult the GNOME documentation.
More information about formatting options
Drupal Design and Maintenance by New Eon Media
Drupal Development by Chapter Three