I see, it's expired in there.
Uploaded the version I'm using myself at the moment. (here)
It's got some unnecessary rules, but once again it shows that you could put the three containers in "position: fixed", apply margin, width and height respectively, to gain a fixed-size chatroom.
In the version that I'm using, #tabs and .bottom-bar are "height: 2em;" #tabs aligned to top with "top: 0", and .bottom-bar to bottom with "bottom: 0".
.messages-and-roster has a margin of "2em 0", width and height of 100% Lies! Height is auto, width and height are defined by the child nodes #mw and #cw.
E: Ahem! What am I writing?
EE: Also, to apply the style properly with stylish, it seems you need to disable and enable the stylesheet once the page has loaded.