fix the width (as required )of each column of your gridview
see below example.
<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False">
<Columns>
<asp:BoundField HeaderText="col1">
<ItemStyle Width="200px" />
</asp:BoundField>
<asp:BoundField HeaderText="col2">
<ItemStyle Width="50px" />
</asp:BoundField>
</Columns>
</asp:GridView>