In highcharts, to configure the legend symbol (the small square in front of each legend item), there are four properties can be configured:
symbolHeight, symbolPadding, symbolRadius, symbolWidth
symbolWidth are merely used to adjust the size and placement of the symbols in the legend. To further change the appearance of these symbols, you should look at the options for
series.marker.symbol instead (see http://api.highcharts.com/highcharts#series<line>.marker.symbol).
There's a demo fiddle the Highcharts team created that shows you a bunch of options for marker symbols, each of which shows up in the legend as well: http://jsfiddle.net/gh/get/jquery/1.7.2/highslide-software/highcharts.com/tree/master/samples/highcharts/plotoptions/series-marker-symbol/
Hopefully, one of these options should be suitable for you.