I'm at work, but I think I have a method that may help you if you're on FFXI right now:
* Choose two characters on your keyboard; I recommend the R and U keys, for personal reasons, but any keys pair is fine. Performance is based on what job you're currently on anyway.
* Alternate between pressing your two keys, counting each time you press a key (note: this requires the counting ability).
* When the chat bar stops adding the letters that you are typing, whatever number you counted up to should be the max number of characters that can be entered into a chat window.
* You may want to repeat the process before posting your results; many formulas in FFXI require a large sample size to correctly calculate mechanics (although this mostly applies to things with percentages, such as drop rates, double/triple attack rates, proc rates on certain weapons, etc).
* And of course, share your results! It's a community!
Good luck with your research.