You asked how wide the strip. Then asked where it should be placed. Two different questions. For the location of the strip try the golden number. Divide the height of the wall by 1.618. Measure from the top or bottom of the wall and place a strip of masking tape along that line. It doesn't have to be an exact decimal measurement--- a fraction up or down won't look bad. Stand back and look. Then decide if it looks better closer to the top or the bottom of the room. Then decide on the width and color.