Question

explain how size of a planet or stars is measured​

Answer 1

The most common is to measure the apparent angular diameter of the planet – how big it looks against the sky – very precisely using a telescope. Combining this with a measure of its distance (deduced from its orbit around the Sun) reveals the planet's actual size.

Answer 2

Answer:

the size and distance of stars and planes are measured by parallex method.

u can search on Google about the method