Tool Bar and Address Box

Somehow my tool bar at the bottom of my screen has increased in size, and covers the portion of the threads that is used to scroll sideways.

This is probably a simple fix, but for the life of me can't remember how to decrease it.

I have tried to use the cursor and when it turns into a double arrow (both up and down) tried to drag that tool bar down to make is smaller. Doesn't work.

Somebody help me get things back to the way they were.