Angular size
Also called: arcminute, arcsecond, degree, apparent size.
How big something looks, as an angle: degrees (°), arcminutes (′, 1/60°) and arcseconds (″, 1/60′).
We can't measure sky objects in kilometres by eye, so we measure the angle they span. The full Moon is about 31′ (half a degree); Jupiter about 45″; the Andromeda Galaxy over 3°; the components of a close double star can be 1″ apart.
Your hand is a ready ruler at arm's length: a little finger covers about 1°, three middle fingers about 5°, a fist about 10°.
In AstroPilot
Every object page gives its size and compares fields of view with the full Moon.