Hello,
It is not trivial, but possible.
To do it you should modify 'js/source/users.js' file. Take a look at viewOpenCell method of HtmlGenerationUtils object (lines 76 - 104).
There are several pairs of values numbers 640 and 480 (lines 81, 88, 94). They are width and height of operator's chat
window respectively. You can change them to any other values.
More information about changing JavaScript files you can find here:
http://mibew.org/forums/index.php/topic,329.0.html