This what worked for me. set HeaderStyle-Width="5%", in the footer set textbox width Width="15",also set the width of your gridview to 100%. following is the one of the column of my gridview.
<asp:TemplateField HeaderText = "sub" HeaderStyle-ForeColor="White" HeaderStyle-Width="5%">
<ItemTemplate>
<asp:Label ID="sub" runat="server" Font-Size="small" Text='<%# Eval("sub")%>'></asp:Label>
</ItemTemplate>
<EditItemTemplate>
<asp:TextBox ID="txt_sub" runat="server" Text='<%# Eval("sub")%>'></asp:TextBox>
</EditItemTemplate>
<FooterTemplate>
<asp:TextBox ID="txt_sub" runat="server" Width="15"></asp:TextBox>
</FooterTemplate>