You will have to set a fixed size
for the pie chart. Can be a percentage or a pixel value. For example:
plotOptions: {
pie: {
size: 100
}
},
See this fiddle from the API http://jsfiddle.net/gh/get/jquery/1.7.2/highslide-software/highcharts.com/tree/master/samples/highcharts/plotoptions/pie-size/