It's just the abbreviation or a ticker symbol used for identification on the markets. Technically there are no limitations, but what would be the point of them being long? Imagine US dollar with abbreviation USdoll..... If you look at the list of all coins on coinmarketcap, some of the even have 5 letters. That is maybe already too long.
If any of those coins would have been currencies (https://en.wikipedia.org/wiki/Currency) as well, then they would also have an ISO code (for dollar it is USD) which is a standard (norm) and since it is a standard it defines exactly how the code can look like - in this case it is to be comprised of three letters (at least I think).
If any of those coins would have been currencies (https://en.wikipedia.org/wiki/Currency) as well, then they would also have an ISO code (for dollar it is USD) which is a standard (norm) and since it is a standard it defines exactly how the code can look like - in this case it is to be comprised of three letters (at least I think).
So basically there is no standard norm for crypto currency yet so they can have as many symbols as they want.. got it, thanks!