What is Angular size and resolution?

The angular size of an object of height $h$ at distance $D$ is $\theta \approx h/D$ (small angles). The eye distinguishes two points only if their angular separation exceeds its angular resolution, about $\epsilon \approx 3 \times 10^{-4}\,\mathrm{rad}$ (one arcminute): the spacing of the cone cells and diffraction at the pupil both set this limit.