Please refer to the attached screenshot.

Workaround:

public Form1()
{
    InitializeComponent(); 

    this.radScheduler1.ElementProvider = new MyElementProvider(this.radScheduler1);
}

public class MyElementProvider : SchedulerElementProvider
{
    public MyElementProvider(RadScheduler scheduler) : base(scheduler)
    {
    }

    protected override T CreateElement<T>(SchedulerView view, object context)
    {
        if (typeof(T) == typeof(AppointmentElement))
        {
            return new CustomAppointmentElement(this.Scheduler, view, (IEvent)context)as T;
        }

        return base.CreateElement<T>(view, context);
    }
}

public class CustomAppointmentElement : AppointmentElement
{
    protected override Type ThemeEffectiveType     
    { 
        get    
        { 
            return typeof(AppointmentElement);     
        }
    }

    public CustomAppointmentElement(RadScheduler scheduler,
        SchedulerView view, IEvent appointment) : base(scheduler, view, appointment)
    {
    }

    protected override SizeF ArrangeOverride(SizeF finalSize)
    {
        SizeF s = base.ArrangeOverride(finalSize);
        return new SizeF(s.Width - 5,s.Height);
    }
}