Try replace width size to 604 px to 100%
Or you can also check how i did it
its aspx page where i have initialized Grid View and Bind it with my data source & some action added from properties window like onrowediting, onrowupdating, onrowcancelingedit...!!
<asp:gridview id="GridView1" runat="server" autogeneratecolumns="false" xmlns:asp="#unknown">
DataKeyNames="UserId" onrowediting="GridView1_RowEditing"
onrowupdating="GridView1_RowUpdating"
onrowcancelingedit="GridView1_RowCancelingEdit">
<columns>
<asp:templatefield headertext="Name">
<itemtemplate>
<asp:label id="lblName" runat="server" text="<%# Eval("Name") %>"></asp:label>
</itemtemplate>
<edititemtemplate>
<asp:textbox id="txtName" runat="server" text="<%# Eval("Name") %>"></asp:textbox>
</edititemtemplate>
</asp:templatefield>
<asp:templatefield headertext="Edit">
<itemtemplate>
<asp:linkbutton id="lnkEdit" runat="server" commandname="Edit">Edit</asp:linkbutton>
</itemtemplate>
<edititemtemplate>
<asp:linkbutton id="lnkUpdate" runat="server" commandname="Update">Update</asp:linkbutton>
<asp:linkbutton id="lnkCancel" runat="server" commandname="Cancel">Cancel</asp:linkbutton>
</edititemtemplate>
</asp:templatefield>
</columns>
</asp:gridview>