From our viewing perspective on the surface of the Earth, the Sun is almost perfectly eclipsed by our Moon. This means they have the same (relative) size.
The Moon has a radius of around 1,700 kilometres and orbits Earth at an average distance of 380,000 kilometres. The Sun has a radius of approximately 700,000 kilometres and Earth orbits it at a mean distance of almost 150,000,000 kilometres.
Knowing the sizes and orbits of the Sun and Earth/Moon we can form triangles and use trigonometry and/or intercept theorem to calculate the vanishing point, the distance from the Sun (and Moon) where both will have a (relative) physical size of zero.
Surprisingly this vanishing point is not as large as one might imagine.