Hi Rob,
This is different on versions prior to v3.2.1 and after. In the older versions, the length is the max length a string can be, but a string of any length up to that can be used. In the later versions, the string is variable length. I am however not sure of the specifics of this, as the documentation is unclear, I will follow up.
Low Bandwidth Design - Blues Developers is the documentation I was referring to.
Kimball