To reproduce: 
public Form1()
        {
            InitializeComponent();

            Random r = new Random();
            DataTable table = new DataTable();
            table.Columns.Add("ID", typeof(int));
            table.Columns.Add("Name", typeof(string));
            table.Columns.Add("Bool", typeof(bool));
            table.Columns.Add("DateColumn", typeof(DateTime));

            for (int i = 0; i < 10; i++)
            {
                table.Rows.Add(i, "Row " + i, r.Next(10) > 5 ? true : false, DateTime.Now.AddHours(i));
            }

            radMultiColumnComboBox1.DataSource = table;
            radMultiColumnComboBox1.EditorControl.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
            
            radMultiColumnComboBox1.DropDownMinSize = new Size(500, 500);
        }

To work around:
            radMultiColumnComboBox1.DropDownOpening += new Telerik.WinControls.UI.RadPopupOpeningEventHandler(radMultiColumnComboBox1_DropDownOpening);

void radMultiColumnComboBox1_DropDownOpening(object sender, CancelEventArgs args)
{
    radMultiColumnComboBox1.MultiColumnComboBoxElement.MultiColumnPopupForm.MinimumSize =          new Size(500, 500);
}