How do I expand my Files page?

The right rail on the Files tab can be hidden in your Home page or within a space.  To do this in your Home page, click on > at the top left of your Reminders pad, in line with the Files heading.

To do this inside a space, click on > at the top left of the space image, in line with the Files heading.

If you want to show the right sidebar again, simply click on < at the upper right side of the page, underneath the ? icon and in line with the Files heading.