Thank you. That is partially correct and helpful.
I still feel strongly that there should be a method at treelist level that should allow you to set height.
I said it's only partial correct because we also have to set the scrollheight. I will insert the code here for further reference.
Thank you again , you were most helpful.
var tl = $find("<%=tlHierarchy.ClientID %>");
tl._element.style.height = document.documentElement.clientHeight - 150 + "px";
var scrollArea = $get("<%=tlHierarchy.ClientID %>" + '_rtlData');
var header = $get("<%=tlHierarchy.ClientID %>" + '_rtlHeader')
scrollArea.style.height = document.documentElement.clientHeight - header.clientHeight - 160 + "px";