1. Turn the Autogrid OFF.
2. Set the current pen width to zero points. You may have to select the Pens... command to create this as a custom pen width.
3. Go to the Master Slide and draw a rectangle to fill the entire drawing.
4. Use the cursor keys to nudge the rectangle towards the upper left corner of the drawing. Scroll to the lower right corner of the drawing and, if necessary, expand the rectangle to fill the entire drawing. (You may want to zoom in to expand the rectangle with more precision.)
5. Change the fill and pen of the rectangle to the desired color/pattern/gradient combination and send the rectangle to the back if needed.
6. Select Turn on Slide Show.
To create a slide show that doesn't shrink to fit the screen, set the drawing size to 8.89 inches wide and 6.67 inches high.