The ideal putter length sets your eyes 1-2″ inside of the target line to allow you to execute the proper arcing putting stroke while maintaining good posture and balance (See the Path illustration).
If your putter is too long, your setup posture will be too upright with your eyes set too far inside the target line. For a right-handed golfer this results ina path that starts too far inside and putts pushed to the right.
If your putter is too short, your setup posture will be too hunched over with your eyes set too far outside the target line. For a right-handed golfer this results in a path that starts too far outside and putts pulled to the left.