Recent developments in the physics of non-magnetic ferroics identified possible functionality of domain walls in these materials, which specifies the current interest to the internal structure of the walls. In this context, the wall thickness itself can be regarded as an important parameter of walls as functional elements, raising the following questions. How thick can ferroelectric domain walls be? To what extent can their thickness be controlled? In answering these questions, we discuss mechanisms controlling the wall thickness in non-magnetic ferroics for different types of the walls. Besides, we provide an overview of the recent results on anomalously thick ferroelectric domain walls.