[Deff]

<script type="text/javascript" src="http://yandex.st/jquery/1.7.2/jquery.min.js"></script>

<div id=Wrap style="position:fixed;z-index:1200;top:120px;left:0;border:red solid 2px;width:80%;height:430px;max-height:430px;">
aaaaaaaaassssssssssssssssssss<br>
saaaaaaaaaaaaaaaaaassssssssssssssssssss<br>
saaaaaaaaaaaaaaaaaassssssssssssssssssss<br>
saaaaaaaaaaaaaaaaaassssssssssssssssssss<br>
saaaaaaaaaaaaaaaaaassssssssssssssssssss<br>
saaaaaaaaaaaaaaaaaassssssssssssssssssss<br>
saaaaaaaaaaaaaaaaaassssssssssssssssssss<br>
<div id=clear style="border:red solid 1px;width:100%;height:3px"></div>


 <textarea id="MyTex" style="border:blue solid 1px;width:100%;"></textarea>

</div>

<script type="text/javascript">
  var delta = 12;

function resizTextar() {
    heitTex=$('#Wrap').height()-$('#clear').position().top-delta;
    $('#Wrap #MyTex').css({"height":heitTex})
}resizTextar();

</script>

При растягивании div - обращаемся к функции resizTextar()