How does the crystal know where to stop stacking the unit cells?
The ideal crystal is terminated by planes, which are described by 
Miller Index. Some of these planes are more favorable than others, and as a result, the crystals do get macroscopic shape which is different from the shape of unit cell. For some discussion on reasons for different stability, I have found this article: iopscience.iop.org/0022-3719/12/22/036/pdf/jcv12i22p4977.pdf