All of these answers, while correct, are skirting around how it's supposed to work.
In most communities, the length of the weapon has nothing to do with its legality. It will come down to whether it is a dangerous weapon.
Basically, IF you are carrying the sword as a weapon--brandishing it at people, using it fight crime, intimidating people with it--it becomes a dangerous weapon and law enforcement will stop you.
IF you are carrying it from your car into a training facility, wearing it as part of a re-enactment (and no, you can't claim you're a re-enactor...you need to be in a location where there's visibly such going on), are using it in a safe demonstration, etc., you're pretty much good to go anywhere.
It comes down to intent. If your intent is to view it as a weapon, it's a weapon. If it's a prop, tool, training item, etc., being used in the correct context, then it's whatever you say it is, but not a weapon.