Andrew,

The Charts are responsive.  Can you place your chart in a grid with a minimum size?  This will make the bars larger.

If you have limited screen real estate, then you can consult the Highcharts API documentation (http://api.highcharts.com/highcharts).  It looks like ‘pointWidth’ is what you want to set via a Before Render Snippet (http://api.highcharts.com/highcharts/plotOptions.bar.pointWidth).

Thanks,

Bill