Your putter length should be set to mirror your full swing set up.  Ideally that is a posture that will give you optimum balance and the least amount of stress on your body and in particular your back.  You have only one balanced stance width where your hands hang exactly the same for putter.  Set up in that stance width and hold a weight across your chest to determine your ideal balanced posture with the putter as shown in this video.  The set up in this video is for a core swing.

If you are a player who has more spine tilt than knee flex, your putter should be set at a length to mirror that set up.  You will still have the same stance width, just a longer putter.  If you watched the 2011 Masters, the players on the leader board mrrored their full swing set up with their putter.  Charl Schwartzel, Jason Day, Adam Scott and Luke Donald made a core swing with a level shoulder turn. Their spine tilt angles and thigh angles were exactly the same.  Their putter set up was the same as their full swing set up.  KJ Choi and Tiger Woods had more spine tilt than thigh angle, they make an arms and hands swing and their putter set up mirrored their full swing set up.  One set up for irons, woods and putter will create consistencey in your game.