void radGridView1_CellEditorInitialized(object sender, GridViewCellEventArgs e)
{
    if (radGridView1.CurrentColumn is GridViewMultiComboBoxColumn)
    {
        RadMultiColumnComboBoxElement editor = (RadMultiColumnComboBoxElement)radGridView1.ActiveEditor;
        editor.AutoSizeDropDownToBestFit = true;
 
        editor.EditorControl.MasterTemplate.Columns[0].Width = 100;
        editor.EditorControl.MasterTemplate.Columns[1].Width = 200;
        editor.DropDownWidth = editor.EditorControl.MasterTemplate.Columns[0].Width +
            editor.EditorControl.MasterTemplate.Columns[1].Width;
        editor.EditorControl.ViewCellFormatting -= EditorControl_ViewCellFormatting;
        editor.EditorControl.ViewCellFormatting += EditorControl_ViewCellFormatting;
    }
}
Font f = new Font("Verdana", 12);
private void EditorControl_ViewCellFormatting(object sender, CellFormattingEventArgs e)
{
    e.CellElement.Font = f;
}