Angular diameter is how large an object looks in the sky, measured as an angle. The Sun and Moon both span about half a degree (around 32 minutes of arc), while the ISS spans up to about one minute of arc overhead, which is why a telescope is needed to see its shape during a transit.
Understanding Angular Diameter
Working it out
Divide the size by the distance and multiply by 206,265 to get seconds of arc. The ISS, 109 m across at 420 km: 0.109 ÷ 420 × 206,265 ≈ 54″. The same station at 1,000 km, low in the sky: about 22″. The Moon, 3,474 km across at 384,400 km: about 1,860″, or 31′. The rule works for anything far away compared with its size, which covers everything in the sky.
From angles to pixels
To know how big a satellite will look in a photograph, work out the camera's image scale: 206.3 times the pixel size in micrometres, divided by the focal length in millimetres, gives seconds of arc per pixel. A 2,000 mm telescope with 3.76 µm pixels gives about 0.39″ per pixel, so a 54″ space station covers about 140 pixels and the whole 32′ Sun nearly 5,000. Detail starts to show at a few dozen pixels across, which is why transit photographers use long focal lengths.