If you create a WebPart containing a multiline TextBox, the TextBox does not size automatically with the width of the WebPart. Requesting the Width of the WebPart is not an option because its not yet in the clients browser. So the following example shows a way of sizing the TextBox to 100% in the WebPart. tbRemark is a TextBox with the TextMode set to TextBoxMode.MultiLine.
protected override void RenderWebPart(HtmlTextWriter output)
{
output.WriteLine("<style>.StyleRemark { width:100%;height:100; }</style>");
output.WriteLine("<TABLE id=\"Table1\" cellSpacing=\"1\" cellPadding=\"1\" width=\"100%\" border=\"0\">");
output.WriteLine("<TR><TD>");
tbRemark.Text = Remark;
tbRemark.CssClass = "StyleRemark";
tbRemark.RenderControl(output);
output.WriteLine("</TD></TR>");
output.WriteLine("</TABLE>");
}