In order to increase the size of collapsing arrow button firstly you have to increase SplitterWidth:
this.radSplitContainer1.SplitterWidth = 16;
Then you have to change the arrow primitive scale factor with ScaleTransform property:
this.radSplitContainer1.Splitters[0].NextArrow.ScaleTransform = new SizeF(3,3);
this.radSplitContainer1.Splitters[0].PrevArrow.ScaleTransform = new SizeF(3,3);
Please note that HorizontalResizeCursor property was introduced in Telerik UI for WinForms version R1 2019.
Regards,
Dimitar
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