To get the biggest output he can get which is directly proportional to the Rroom's size which by extension directly proportional to the length of his sword. That's how Law's room works. When it's come to Room, the bigger the room is, the stronger is his cutting ability. In Kroom, the longer the room is, the stronger are the shock waves which is the reason why the second Shock Wille was more powerful than the first Shock Wille. If it's the tip that he was worried about, leaving it open in the mid-air would solve the problem.