My best guess is that I have to copy a file across all my chars in order to get the same chat windows size on all of them, I just don't know which one. Anyone care to give me a hand?
If I remember correctly its WoW>WTF>Account>[your account number]>Server>Character>Chat-Cache.txt that you need to copy to which ever toons you want those setting son.
You can also use the addon prat, and set the minimum/maximum sizes for the chat window. If you make the minimums the size you prefer, you can always resize the window to the exact size you want, with the option to enlarge it when you need to (trade spam or whatever), and then slide it right back to your preferred size.