You may arrange your models any way your heart desires, from 30 person wide fronts 1 deep to 1 wide and 15 models back. However ranks must be at least 5 wide in order to gain the rank bonus in combat. Therefore it is usually foolish to make your units narrower than 5. However it is sometimes advantageous to make them wider. For instance if you can get more attacks in as corners count, and if they have bows or something, more models will be able to see to shoot.