|You can only change the size of the window when the center button (to the left of the button with red box with a white X icon on it) at the top right corner of your screen has one icon on it.|
Click once on the center button that has the two icons at the top right corner of your screen - it will then toggle to have one icon on it.
(Clicking it once again will toggle it to have two icons on it.)
Hold your cursor over the edge of the window on one side or the other so it changes to a horizontal double headed arrow (you may see only one side of it at first) , hold down the left mouse button, and move the mouse to drag the edge of the window to make the window smaller horizontally.
The same for the bottom of the window, except it's a vertical double headed arrow (you may only see the top of it at first) .
Hold the cursor over the top blue bar in the window, hold down the left mouse button, move the mouse to drag the whole smaller window to where you want it.