浏览状态下的列宽度可以这样设置：
Column.ItemStyle.Width = 100;

编辑状态下的列宽度可以这样设置：

private void DataGrid1_EditCommand(object source, System.Web.UI.WebControls.DataGridCommandEventArgs e)
{
    DataGridItem line = dbgCustomers.Items[e.Item.ItemIndex];
    TextBox tb1 = (TextBox)line.Cells[0].Controls[0];
    TextBox tb2 = (TextBox)line.Cells[1].Controls[0];

    tb1.Width = Unit.Percentage(100);
    tb2.Width = Unit.Percentage(100);
    tb2.TextMode = TextBoxMode.MultiLine;
}