What is the boundary of the solar system? It is the kind of cosmic region that separates the “solar wind” (a constant stream of protons, electrons and alpha particles emitted by the sun) and “interstellar space” (ISM). It’s called the heliopause. It is also the border of the heliosphere – a bubble of space, where the Sun’s magnetic field and particle emissions are located.
As for the size of the border, it is calculated in astronomical units (AU). One au is the average distance between the Sun and the Earth (149,597,870,700 meters). So, the heliosphere is located at about 120 AU. from the Sun in the direction facing the interstellar wind, and in the opposite direction – at least 350 AU.