  First sketch of black hole from M87 galaxy was obtained by Event Horizon
Telescope, recently. As appearance of black hole shadow reflects space-time
geometry of black holes, observations of black hole shadow may be a promising
way to test general relativity in strong field regime. In this paper, we focus
on angular radius of spherical black hole shadow with respect to freely falling
observers. In the framework of general relativity, aberration formulation and
angular radius-gravitational redshift relation are presented. For the sake of
intuitive, we consider parametrized Schwarzschild black hole and
Schwarzschild-de Sitter black hole as representative examples. We find that the
freely in-falling observers would observe finite size of shadow, when they go
through inner horizon. For observers freely falling from the outer horizon of
Schwarzschild-de Sitter black hole, we find that the angular radius of the
shadows could increase even when the observers move farther from the black
hole.
