public RadForm1()
{
    InitializeComponent();
    this.radGridView1.FilterPopupInitialized += RadGridView1_FilterPopupInitialized;
}

private void RadGridView1_FilterPopupInitialized(object sender, FilterPopupInitializedEventArgs e)
{
    RadListFilterPopup popup = e.FilterPopup as RadListFilterPopup;
    popup.MinimumSize = new Size(300, 300);
    popup.PopupOpened += Popup_PopupOpened;
}

private void Popup_PopupOpened(object sender, EventArgs args)
{
    RadListFilterPopup popup = sender as RadListFilterPopup;
    popup.LocationChanged += Popup_LocationChanged;
    popup.Location = new System.Drawing.Point(MousePosition.X, MousePosition.Y);
}
private void Popup_LocationChanged(object sender, EventArgs e)
{
    RadListFilterPopup popup = sender as RadListFilterPopup;
    if (popup != null)
    {
        popup.LocationChanged -= Popup_LocationChanged;
        Screen screen = Screen.FromPoint(new System.Drawing.Point(popup.Left, popup.Top));
        if (popup.Left + popup.Width > screen.WorkingArea.Right)
        {
            popup.Left = screen.WorkingArea.Right - popup.Width;
        }
    }
}