  Topological insulator surfaces in proximity to superconductors have been
proposed as a way to produce Majorana fermions in condensed matter physics. One
of the simplest proposed experiments with such a system is Majorana
interferometry. Here, we consider two possibly conflicting constraints on the
size of such an interferometer. Coupling of a Majorana mode from the edge (the
arms) of the interferometer to vortices in the centre of the device sets a
lower bound on the size of the device. On the other hand, scattering to the
usually imperfectly insulating bulk sets an upper bound. From estimates of
experimental parameters, we find that typical samples may have no size window
in which the Majorana interferometer can operate, implying that a new
generation of more highly insulating samples must be explored.
