You have to set it like this
<asp:GridView ID="gvDemo2" runat="server" AutoGenerateColumns="false">
<AlternatingRowStyle Width="60px" />
<Columns>
<asp:BoundField DataField="FileId" HeaderText="Name" ItemStyle-Width="100" />
<asp:BoundField DataField="Name" HeaderText="Name" ItemStyle-Width="100" />
<asp:BoundField DataField="Number" HeaderText="Number" ItemStyle-Width="100" />
<asp:BoundField DataField="Email" HeaderText="Email" ItemStyle-Width="100" />
<asp:BoundField DataField="Date" HeaderText="Date" ItemStyle-Width="100" />
<asp:BoundField DataField="Message" HeaderText="Message" ItemStyle-Width="100" />
</Columns>
</asp:GridView>
ItemStyle-Width="100" will set the column width.