We are using a touchscreen.
At highchart there are two small buttons over the scrollbarbutton.

http://jsfiddle.net/gh/get/jquery/1.7.2/highslide-software/highcharts.com/tree/master/samples/stock/scrollbar/height/

I am not able to change the dimensions of this small buttons which are used to change the timeselection.
This property can also be combined with the dimension of the scrollbar { height: XX, width: XX}