The Schwarzschild radius of a black hole with the Sun’s mass is approximately 3 kilometres.
The Schwarzschild radius is the radius of the event horizon for a nonrotating, uncharged black hole. Karl Schwarzschild derived the relevant solution to Einstein’s field equations in 1916, shortly after general relativity was published. It shows that concentrating a given mass inside a sufficiently small radius creates a black hole.
A solar-mass black hole would therefore have an event-horizon diameter of about 6 kilometres. The Sun is nowhere near compact enough to become a black hole naturally; its radius is about 696,000 kilometres. The formula scales directly with mass, so a black hole ten times as massive as the Sun would have a Schwarzschild radius of roughly 30 kilometres.
This radius is not the size of a material object. It describes a spacetime boundary, and the calculation applies exactly only to an idealized nonrotating, uncharged black hole.