I wrote a script that did just this before this thread was created - I called it iChat+. My script automatically calculates the windows width before it resizes so the width remains the same after a resize... none of the funky variable addition stuff, becuase in my opinion it should "just work". Please, feel free to take a look at the code on pastebin, use it at your will to modify or roll your own solution. The code is not well commented, and does not come with any documentation. (Although the variable names should be fairly self explanitory)
PLEASE NOTE: There is a reason I never released this code - it doesn't "just work".
To elaborate, I use iChat's Jabber feature to connect with Facebook chat. This is the only thing I use iChat for. It appears that Facebook's chat solution sends login and logout status updates at regular intervals, meaning that when Facebook pushes this info to iChat you will get multiple buddies logging in and out simulatenously. When the script executes, AppleScript will hijack iChat and will not allow you to execute other functions while the AppleScript is executing. In short, you cannot send messages (or perform any other function) momentarily while buddies are logging in or out.
This issue is less visible when only one buddy logs in or out at a time.
This is not a problem with the script, it is an issue with Apple's implemenation of AppleScript within iChat and Facebook's use of Jabber. I have no doubt that this error will also occur with the script @tiagofromcoimbra posted above.