well, in theory a minifig scaled to human size is around what 6foot tall at a 1/48 scale. The sword is 1" long so at at minifig scale it would theoretically be 4 feet long. By most comparisons of the blade and body size of Link, I would say a 4 foot blade is relatively accurate for the Master Sword. In the Game OoT, the photo of adult Link holding the Master Sword with it lowered to the ground and holding his shield, I would say a 4 foot blade is a pretty good representation, BUT that i suppose does depend on which Zelda game you are wanting to mimic as there are more than enough discrepancies to say that we are all wrong and all right at the same time.
So figure out which game you wanna work off of, and then you can figure out if you wanna go with a custom sword or one that is easily available that just needs a paintjob