The esc key on your keyboard will also exit full screen mode. it's usually the key at the top left of the keyboard, but depending on your keyboard, it might be someplace else -- also sometimes called the 'escape' key.
There is also a "Classic Layout" that you might try -- open Mail Preferences and click on the Viewing tab along the top -- then click the checkbox labeled "Use classic layout".
In any event, you can use the screen cursor and your trackpad/mouse to vary the width of some columns -- just watch the screen closely as you move the cursor from the left edge to the right edge of the mail window and notice how the cursor changes from a simple arrow pointing the the upper left to an error that points in both directions - thats a hint that you can now click and drag to vary the width of the column to the left of the cursor.
Good luck...