Cannot Change The Width Of Button In Gridview

I have a select button in gridview, and I want to change the width of the button. I have tried the ItemStyle-Width . But it does not work.

<asp:CommandField ButtonType="Button" ItemStyle-Width = "20px" ShowSelectButton="True" >
</asp:CommandField>

And I tried the following in

protected void grdHeading_RowCreated(object
sender, GridViewRowEventArgs e)
if (e.Row.RowType ==
DataControlRowType.DataRow)
{
e.Row.Cells[0].Width = 40;
e.Row.Cells[1].Width = 40;
e.Row.Cells[2].Width = 20;
}
}

But the width of the Select button still not changed. How can I change the width of the select button?