Left unchecked the fast growing Lleylandii for example would grow to a massive tree over 60ft high. If trimmed reguarly (at least once a year) it can be contained. If you 'take the top out' it will continue to grow outward and if left unchecked, very soon new upward growth would be produced and away it would go again. Regular trimming is required to keep this type in shape.
There are however, many different types of conifer that are slower growing and would be easier to maintain.