Yep there was a rule a couple of years ago, but they took it out again.
I do how ever have a funny story as I went to MK to do dressage once and the judge pulled me up and said my whip was to long for BD rules, I siad in a quick reply are you sure its not that my pony is to short lol ! To which she fell about laughing lol !
there is no max length - but it cant be so long that it interfers with other riders in the warmup. My whip is 115cm which is plenty long enough for competing with. My one i ride with daily is 140cm
And i would never compete with it as id probably end up taking someones horses eye out or something
I was once called to the judges box after my test because my whip was too long. Dont know how long it actually was but it was longer than my any of my other whips - I just dont carry one anymore in a test!