  The non-trivialness of a topological insulator (TI) is characterized either
by a bulk topological invariant or by the existence of a protected metallic
surface state. Yet, in realistic samples of finite size this non-trivialness
does not necessarily guarantee the gaplessness of the surface state. Depending
on the geometry and on the topological indices, a finite-size energy gap of
different nature can appear, and correspondingly, exhibits various scaling
behaviors of the gap. The spin-to-surface locking provides one of such
gap-opening mechanisms, resulting in a power-law scaling of the energy gap.
Weak and strong TI's show different degrees of sensitivity to the geometry of
the sample. As a noteworthy example, a strong TI nanowire of a rectangular
prism shape is shown to be more gapped than that of a weak TI of precisely the
same geometry.
