tickSize property
Specifies the size for tick.
Specifies the size for the major ticks in the SfSlider,
SfRangeSlider, and SfRangeSelector.
Major ticks is a shape which is used to represent the major interval points of the track.
Defaults to Size(1.0, 8.0).
This snippet shows how to set major tick size in SfRangeSliderThemeData.
SfRangeValues _values = SfRangeValues(4.0, 8.0);
Scaffold(
    body: Center(
        child: SfRangeSliderTheme(
            data: SfRangeSliderThemeData(
                tickSize: Size(3.0, 12.0),
            ),
            child:  SfRangeSlider(
                min: 2.0,
                max: 10.0,
                interval: 2,
                showTicks: true,
                values: _values,
                onChanged: (SfRangeValues newValues){
                    setState(() {
                        _values = newValues;
                    });
                },
           )
       ),
   )
)
Implementation
final Size? tickSize;