To reproduce:
radCollapsiblePanel1.CollapsiblePanelElement.HeaderElement.HeaderButtonElement.MinSize = new Size(100, 100);

Workaround:

 //resize the button
                this.menue.CollapsiblePanelElement.HeaderElement.MinSize = new Size(0, 50);
                this.menue.CollapsiblePanelElement.HeaderElement.HeaderButtonElement.MinSize = new Size(50, 50);
                this.menue.CollapsiblePanelElement.HeaderElement.HeaderButtonElement.MaxSize = Size.Empty;
                //refresh the control
                this.menue.EnableAnimation = false;
                this.menue.IsExpanded = !this.menue.IsExpanded;
                this.menue.IsExpanded = !this.menue.IsExpanded;
                this.menue.EnableAnimation = true;