Hi.Your problem is that td will not give percentage out.If you use pixels you will see that it expands.
What i think you can do is, since your are using rowspan, try to set the exact same rowspan of the td to the textbox rows like this:
These are assumptions i haven't actually tested an expanding textbox inside a td.Let me know how this turns out.
<table cellpadding="0" cellspacing="0" style="width:100%;height:100%;background:#ccc;">
<td rowspan="14"><asp:TextBox ID="AppInfoTextBox" TextMode="MultiLine" Rows="14" style="width:100%" runat="server" /></td>