Workaround:

private void radGridView1_CellEditorInitialized(object sender, Telerik.WinControls.UI.GridViewCellEventArgs e)
{
    GridBrowseEditor browseEditor = e.ActiveEditor as GridBrowseEditor;
    if (browseEditor!=null)
    {
        browseEditor.EditorElement.MinSize = new Size(0,18);
        GridBrowseEditorElement el = browseEditor.EditorElement as GridBrowseEditorElement;
        el.TextBoxItem.TextBoxControl.MinimumSize  = new Size(0, 13);
    }
}