textbox width constrained VS2017 default web side

Hi

I have created the default web site from the menu of VS2017 using file - new - web site.

On the default.aspx web page, I want to place a text box that will span 100% of the web page.  But, the best width I can get is about 25% of the web page in a browser.  I have removed all the code provided within the content tag and have only a textbox.  Here is the code.  This is the only change from the default web site.

<asp:Content ID="BodyContent" ContentPlaceHolderID="MainContent" runat="server">

                <asp:TextBox ID="TextBox3" runat="server"

                    width="100%">

                </asp:TextBox>

</asp:Content>

Any suggestions?  Thanks for any help.