Click on the plot area and you will see light blue dots appear around the plot. Data on the horizontal axis is outlined in purple, while data for the series is outline in blue.
Move the mouse over one of the dots and when the double arrow appears, click and drag the border of the plot to the desired size. Light blue dotted lines will show the new border location. Notice that the plot area includes the area around the axes so if you increase the plot area, the axes will move with it.
Changing the size of the plot area may also require you to format the axes, the legend, and chart and axis titles, if you include them. Click on the links if you want to jump ahead (or back) to these topics.
As with formatting the chart area, formatting the plot area allows you to change visual features of the plot area. Scroll the mouse over the gridlines so that the cursor says Plot Area (give it a second) and right click. The Format Plot Area dialogue box will appear. You can also get to this dialogue box by clicking on Plot Area in the Background group under the Layout tab and selecting More Plot Area Options.
Alternatively, you can reach this dialogue box by going to the Format tab or the Layout tab under Chart Tools and selecting Plot Area from the dropdown menu found at the top of the Current Selection group (far left of the Ribbon under both tabs). Then select Format Selection.
The quickest way to open the dialogue box is to double click on the plot area.
You can alter the Fill, the Border Color, and the Border Styles of the plot area. You can also add Shadow, Glow and Soft Edges, or 3-D Format to the plot area. As with formatting the visual aspects of the chart area, follow the standards of the discipline, the requirements of the project, or your own intuition.