public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        this.radDateTimePicker1.DateTimePickerElement.PropertyChanged += DateTimePickerElement_PropertyChanged;
    }
    Size initialPopupSize;
    private void DateTimePickerElement_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        if (e.PropertyName == "IsDropDownShown")
        {
            RadDateTimePickerCalendar calendar = this.radDateTimePicker1.DateTimePickerElement.GetCurrentBehavior() as RadDateTimePickerCalendar;
            if (calendar.PopupControl.MinimumSize != initialPopupSize && initialPopupSize != Size.Empty)
            {
                return;
            }

            float scale = 0;
            float dpi = this.CreateGraphics().DpiX;

            if (dpi == 96)
            {
                scale = 1f;
            }
            else if (dpi == 120)
            {
                scale = 1.25f;
            }
            else if (dpi == 144)
            {
                scale = 1.5f;
            }
            else if (dpi == 192)
            {
                scale = 2f;
            }

            Size popupSize = calendar.PopupControl.Size;
            Size newSize = new Size((int)(popupSize.Width * scale), (int)(popupSize.Height * scale));
            calendar.PopupControl.MinimumSize = newSize;
            initialPopupSize = popupSize;
        }
    }
}