Hi @AP_9587236,
You were correct. Naming new users like Things cannot exceed 255 characters.
I confirmed that putting 256 characters leads to a warning and cannot be saved from composer.
So the 255 length is a constraint coded at application level not a constraint from database. It's just the way how ThingWorx is designed.
I think 255 is enough length for naming a user or a Thing though. Any special reason to have a name more than 256 characters?