  We study how the surface states in the strong topological insulator
Bi$_2$Se$_3$ are influenced by finite size effects, and compare our results
with those recently obtained for 2D topological insulator HgTe. We demonstrate
two important distinctions: \textit{(i)} contrary to HgTe, the surface-states
in Bi$_2$Se$_3$ display a remarkable robustness towards decreasing the width
$L$ down to a few nm, thus ensuring that the topological surface states remain
intact, and \textit{(ii)} the gapping due to the hybridization of the surface
states features an oscillating exponential decay as a function of $L$ in
Bi$_2$Se$_3$ in sharp contrast to HgTe. Our findings suggest that Bi$_2$Se$_3$
is suitable for nanoscale applications in quantum computing or spintronics.
Also, we propose a way to experimentally detect both of the predicted effects.
