In a way it is harmful, but on the other end its not. If you read the manual or even check online, 13" is a recommended size becaise suspensions are made according to this size or the vice versa. But there is no harm even if you put 15" on them until and unless the car is not carrying more weight, or you are not driving on a hilly/broken roads etc which can effect suspesions seriously.
If you want 14" on them, you will need to change your driving habbits.