To reproduce:

DataTable dt = new DataTable();
for (int i = 0; i < 10; i++)
{
    dt.Columns.Add("Column " + i);
}
for (int i = 0; i < 1000; i++)
{
    dt.Rows.Add(i);
}

this.radMultiColumnComboBox1.DataSource = dt;
this.radMultiColumnComboBox1.DisplayMember = "Column 0";
this.radMultiColumnComboBox1.DropDownSizingMode = SizingMode.UpDown;
this.radMultiColumnComboBox1.AutoSizeDropDownToBestFit = true;
this.radMultiColumnComboBox1.EditorControl.LoadElementTree();
this.radMultiColumnComboBox1.MultiColumnComboBoxElement.DropDownAnimationEnabled = false;
this.radMultiColumnComboBox1.MultiColumnComboBoxElement.ShowPopup();


The columns are autosized but the popup is not sized correctly.