Hi,

You can resize the actual pie part of the chart by selecting the
plotarea (square box bounding the slices).

Select the chart and use the Up/Down cursor key to cycle thru the chart
elements. Once the plotarea is selected you can drag to resize.

Or use the chart toolbar to select the plotarea.

Or to click it, image the pie is in a box so click a corner area close
to the slices at roughly the 2,5,7 or 11 o'clock positions.

Cheers
Andy