In the default control template of the RadSlider, the Track element is represented by a ContentControl control. When using the Xaml version of the assemblies, it would require extracting the TrackStyle, in order to base the custom one on it. An alternative would be to use the ChildrenOfType method to retrieve the ContentControl with x:Name="Track" and apply the needed modifications.
Both of these approaches could be a bit overwhelming when only the Width/Height properties of this Track element would need to be updated.
We could expose additional properties for changing the Width/Height values of this ContentControl (x:Name="Track").