Retaining the size of the drop down list

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C#

ASP.NET

I have 2 drop down lists. When i select 1st value from the 1st ddl, the 2nd ddl gets disabled. When i select 2nd value from 1st ddl, the 2nd ddl enables. This just what I needed. But there is a problem. When the 2nd ddl gets disabled, its size increases automatically & I don't want that. T have used table, update panel, trigger, content panel, still its not working.
I am attaching the portion of the code. Kindly help.

<td>
<label>Activity&nbsp;</label>
</td>
<td>
<asp:UpdatePanel ID="Update_Panel1" runat="server">
<contenttemplate>
<asp:DropDownList ID="ddl_Activity1" runat="server" AutoPostBack="True" 
class="w100" onselectedindexchanged="ddl_Activity1_SelectedIndexChanged">     
<asp:ListItem Text="Create Permanent Water Holes" Value="1">
<asp:ListItem Text="Improve Water Storage" Value="2">

</contenttemplate>
<triggers>
<asp:AsyncPostBackTrigger ControlID="ddl_SubActivity1" EventName="SelectedIndexChanged"/>
</triggers>

</td>
<td>
<asp:label ID="lbl_SubActivity1" runat="server">Sub&nbsp;Activity
</td>
<td>
<asp:UpdatePanel ID="Update_Panel2" runat="server">
<contenttemplate>
<asp:DropDownList ID="ddl_SubActivity1" runat="server" 
class="w100" AutoPostBack="True">
<asp:ListItem Text="Desilting" Value="1">
<asp:ListItem Text="Deepening" Value="2">
<asp:ListItem Text="Repairing" Value="3">

</contenttemplate>
<triggers>
<asp:AsyncPostBackTrigger ControlID="ddl_SubActivity1" EventName="SelectedIndexChanged"/>
</triggers>
    
</td>