How to increase TextBox height using JavaScript in ASP.Net

hi

in view.aspx os textbox that bind from database:

<asp:TextBox ID="Txtdes" runat="server" CssClass="TxtdesNM" TextMode="MultiLine"></asp:TextBox>

 css:

.TxtdesNM {
    float: right;
    width: 975px;
    height: 198px;
    border: 1px solid #afafaf;
    color: #6e6e6e;
    direction: rtl;
    text-indent: 10px;
    resize: none;
}

as you see textbox's height is 198px I want if text be morethan this height it doesn't put scrollbar for textbox I want it increase textbox's height automatically...

How I can do it?

best regards

neda