There is no easy way to size the
plot area of a chart to have the same height/width ratio. And
even once you managed to get the ratio set resizing the chart
will undo your hard work.
This trick will false the plot
area to have the same height/width ratio and maintain that ratio
when resizing the chart.
The charts on the right show the resulting aspect ratios after
widening the chart areas.
The trick is you add an additional series and
plot it as a pie chart. The chart will then automatically make
the plotarea aspects equal and maintain the ratio when resizing
the chart
The workbook contains detailed instructions
on how to set up the chart