There doesn't seem to be a length limit for usernames. I was able to set my username to TestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTest (though usernames longer than 254 characters seem to trigger an internal server error).
Hopefully this is the right place to report this issue!
There doesn't seem to be a length limit for usernames. I was able to set my username to
TestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTestTest
(though usernames longer than 254 characters seem to trigger an internal server error).Hopefully this is the right place to report this issue!