Containers with drain holes which allow excessive water to leave is vital for the health of Haworthias. The number and size of holes can be arbitrary since they make no difference in the volume of water drained.
Container porosity is also an important factor. A container made by a porous material (for example, unglazed clay-based ceramics) is air permeable and evaporates a large amount of water through the sides. This causes the tip roots reaching the sides to be desiccated, and therefore inhabits the root growth nearby. In most cases it is better to use a container made by a non-porous materials (for example, plastic and glazed ceramics), except that a rapid water evaporation is needed to adapt a cold or humid climate.
With a growing medium which holds little or no perch water table, there is no upper limit to container size. The lower limit should make at least ~50% room for the roots to grow, otherwise the plant growth will stunted. Some Haworthia species like H. truncata have deep roots and are fitted better in deep containers.