We can fix the width by fixing TR and TD width, example
<table>
<tr style="border-width: 1px; width: 650px; border-color: Black;">
<td width="350px" height="50px">
<asp:TextBox ID="txtFirstName" runat="server" Text='<%# Eval("FirstName") %>' ToolTip='<%# Eval("FirstName") %>'
Enabled="true" ReadOnly="true" BorderStyle="None" Width="100px" Height="50px"
Font-Bold="True" ForeColor="#000099"></asp:TextBox>
</td>
</tr>
</table>