Hi Inung Huang,
Sounds like your dragging the chart data instead of the plot area.
To select the plot area you need to click just outside if the pie itself.
Imagine the pie is in a box. Click at the 5 'clock position. Once you have the plot area you can increase the size.
Another way to highlight the plot area is to click the chart and then use the up/down cursor keys to cycle through the chart elements.