Are you sure you want it to be three times a Blockhead's height? That's 7.95 TU, or 15.9 studs. A
longsword, which is, y'know, a long sword, has a length (blade + hilt) of 39-51 inches, or about three to four feet. The average rule of thumb is that one stud is one foot. Your sword, by that same metric, would end up at a staggering sixteen feet long - definitely impossible to hold, and would look really silly in-game no matter how you animated it.
Your choice, though.