First I tried putting in this, which is exactly 500 characters:

Then I tried putting in this, which is 496 characters (and still got the error):

Then, I put in this and was able to save:

Note that my previous signature had the • character in it and was able to save, so it does accept it. Previous signature was:

My, non-coding-savvy guess is that it is counting special characters like • as more than one character. How many, I am not sure.