I did a trial account to test reward quests on live ... text of the Story was much longer than we supported so I did a lot of testing, playing around with various limits, and added a hard coded limit for Story and FInishText to be 1000 characters. It will vutoff around 990, sometimes 995, but more text does not cause any issues.
I also added a hardcoded 255 limit to StepText. Going over this limit would not crash the client but it would corrupt the Reward Quest dialog.
I am finished with DataQuests for now unless someone comes up with an important feature we are missing.