Set Width of GridView TextBox in Edit mode

To modify the width of a TextBox (inside the GridView control) when the GridView is in Edit Mode, use any of the two approaches:

Using ControlStyle

<asp:GridView ID="gvCustom" runat="server">
    <Columns>
        <asp:BoundField DataField="EmpID" HeaderText="EmployeeID" >
            <ControlStyle Width="150" />
        </asp:BoundField>
    </Columns>
</asp:GridView>

Use ControlStyle CSS Class (Recommended)

<style type="text/css">
.cssWdth
{
    width: 150px;
}
</style>

and in the GridView

<asp:GridView ID="gvCustom" runat="server">
    <Columns>
        <asp:BoundField DataField="EmpID" HeaderText="EmployeeID"
        ControlStyle-CssClass="cssWdth" />
    </Columns>
</asp:GridView>

Check some additional GridView Tips and Tricks over here