Hi,
Changing size of editor on client is not supported feature. However, if you look at generated html then you may find that editor renders outer <table> which id matches with its ClientID. So, as previous suggestion said, you may set its height.
<
script type="text/javascript">
function resizeEditor()
{
var table = document.getElementById("WebHtmlEditor1");
table.style.height = "500px";
}
</script>
<input type="button" value="resize" onclick="resizeEditor()" />
<ighedit:WebHtmlEditor ID="WebHtmlEditor1" runat="server" ...