In the 'Open' box have a look at the dropdown options for the View. At the top right,
in the row that reads: Look in: My Documents: the last icon before 'Tools' is a blue square with tiny icons on.
Click on that for the drop down choices, it is probably set for 'preview' so just click on 'details' and the split screen will go away.
You can get access to the bottom right corner of the 'Open' box by grabbing the top blue bar - left mouse button and hold it down while dragging - drag it over to the left and off the screen until the right edge of Open box is showing.
On mine I can resize the Open box by dragging any of the borders ( you get a 2 headed arrow when you move the mouse cursor over the borders)