The apparent diameter of the Sun is the angle from which it is seen from the position of the observer.
In astronomy we do not measure the apparent size of celestial objects in meter, but in degrees of angle. More the Sun is far of a planet and more its size will appear small and vice versa more it is close and more it is great. For example, on Earth, the apparent diameter of the full Moon is substantially equal to that of the Sun, i.e. a half-degree angle (there are 360 degrees in a circle). This is why the lunar disc perfectly covers the solar disk at the moment of total solar eclipses.
How to measure the apparent diameter of the Sun as seen from the sky of a planet?
The sky is a sphere of 360°.
When you look at the sky, you have above you, a hemisphere of 180° where the stars shine. On this dome of 180°, the size of the Sun represent only a half-degree, it is its apparent size.
You could align on horizon, 720 suns side by side to get around the horizon, i.e. 360°. If the sun seems to us more than one half-degree this is because we are the victim of an optical illusion. This interpretation of the size of the Sun by our brain is wrong. If you place your thumb outstretched arm, in front of the Sun or the Moon, their apparent size is half of your thumb. We now understand that their apparent size is really small as your thumb is only one degree from the celestial circle of 360°. Using the formula for calculating the apparent diameter of the sun from the planets, is how our star is seen from Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto.
nota: simplifying the calculation of the apparent diameter of the sun as seen from the planets, is a function of the actual distance (D) and the actual diameter (d) of the sun, that is d/D.
Image: Apparent size of the Sun as seen from Earth