The experimental domain size scaling law in epitaxial BiFeO3 films shows a different behavior from predictions of the conventional elastic domains: the (101)-type 71° domains are much wider than that of (100)- type 109° despite the larger domain-wall energy in (100) boundary. A phenomenological analysis for rhombohedral BiFeO3 film is proposed, and it reveals that both the depolarizing energy and the elastic energy are indispensable for the equilibrium domain structures. With the increase in the asymmetrical electrostatic boundary on the film surfaces, the dominant domain scaling mechanism changes from electrostatic-dependent domain structure to elastic-dependent one, which is consistent with the experimental data. The present results highlight the general role of depolarizing field in rhombohedral domain structures.