If you are using TemplateFields & Binding in your .aspx file you can control the column width using the following;
<asp:templatefield>
<headerstyle width="10%" />
<itemtemplate>
<asp:label id="lblField" runat="server" text="<%# DataBinder.Eval(Container, "DataItem.ColumnName") %>"><asp:label>
</itemtemplate>
</asp:templatefield>
If you are doing it in the code behind it is simply a matter of getting the grid view after binding & setting the column widths as required
Kind Regards