how to set a width to a gridview column while in update mode?

I have:
<asp:GridView ID="GridView1" runat="server" > <Columns> ... <asp:BoundField DataField="po_no" HeaderText="po_no" SortExpression="po_no" itme />
... </Columns> </asp:GridView>
when in update mode, the column is necessarily too wide.  How can I set a width to it?  I use ItemStyle-Width="50px", but it only applies to view mode; no effect on update.
Thanks.