This is the true size of Jupiter relative to Earth
But have you ever wondered how we can measure the size of the planets in our star system with great accuracy? Well, there are several ways in which the diameters of the planets can be measured. The most common and simplest is to measure the planet's apparent angular diameter (how large it is in Earth's sky) very precisely using a telescope. Combining this with measuring its distance reveals the planet's actual size.
As for distances, we can calculate the distance of nearby objects around our planet, either by deducing them from their orbit around the sun or by a simple law (distance = speed * time), and by sending very concentrated light with a known wavelength and frequency (such as a laser or radar waves) and by calculating the speed Light is 300,000 km/s, and knowing the time in which the light will arrive, as well as knowing the amount of light bouncing off the object, the distance can be calculated with ease.
Source: websites