public class CustomRadDropDownListElement : RadDropDownListElement
{
    protected override RadDropDownListEditableAreaElement CreateTextEditorElement()
    {
        return new CustomListEditableAreaElement(this);
    }
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadDropDownListElement);
        }
    }

}
public class CustomListEditableAreaElement : RadDropDownListEditableAreaElement
{
    public CustomListEditableAreaElement(RadDropDownListElement owner) : base(owner)
    {

    }
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadDropDownListEditableAreaElement);
        }
    }
    protected override SizeF MeasureOverride(SizeF availableSize)
    {
        var elementsDesiredSize = base.MeasureOverride(availableSize);
        SizeF clientSize = this.GetClientRectangle(availableSize).Size;


        SizeF desiredSize = this.Layout.Measure(clientSize);
        // This measures the size of the underlying text box:

        return this.CalculateDesiredSize(availableSize, desiredSize, elementsDesiredSize);
    }
}