The apparent radius of something residing in a Schwarzschild metric, when viewed from infinity is given by $$ R_{\rm obs} = R \left(1 - \frac{R_s}{R}\right)^{-1/2}\ ,$$ where $R_s$ is the Schwarzschild radius $2GM/c^2$.

This enlargement is due to gravitational lensing and the formula is correct down to the "photon sphere" at $R =1.5 R_s$.

Most of the light in the EHT image comes from the photon sphere. It is therefore observed to come from a radius $$ R_{\rm obs} =\frac{3R_s}{2}\left(1 - \frac{2}{3}\right)^{-1/2} = \frac{\sqrt{27}}{2}R_s\ .$$

There are small (<10%) corrections to this for a spinning black hole governed by the Kerr metric.

The apparent radius of something residing in a Schwarzschild metric, when viewed from infinity is given by $$ R_{\rm obs} = R \left(1 - \frac{R_s}{R}\right)^{-1/2}\ ,$$ where $R_s$ is the Schwarzschild radius $2GM/c^2$.

This enlargement is due to gravitational lensing and the formula is correct down to the "photon sphere" at $R =1.5 R_s$.

Most of the light in the EHT image comes from the photon sphere. It is therefore observed to come from a radius $$ R_{\rm obs} =\frac{3R_s}{2}\left(1 - \frac{2}{3}\right)^{-1/2} = \frac{\sqrt{27}}{2}R_s\ .$$

There are small (<10%) corrections to this for a spinning black hole governed by the Kerr metric.

A sketch which illustrates why gravitational lensing leads to an enlarged image can be found in Pela's answer to a related question.

The angular diameter of something residing in a Schwarzschild metric, when viewed from infinity is given by $$ R_{\rm obs} = R\left(1 - \frac{R_s}{R}\right)^{-1/2}\ ,$$ where $R_s$ is the Schwarzschild radius $2GM/c^2$.

This enlargement is due to gravitational lensing and the formula is correct down to the "photon sphere" at $R =1.5 R_s$.

Most of the light in the EHT image comes from the photon sphere. It is therefore observed to come from a radius $$ R_{\rm obs} =\frac{3R_s}{2}\left(1 - \frac{2}{3}\right)^{-1/2} = \frac{\sqrt{27}}{2}R_s\ .$$

There are small (<10%) corrections to this for a spinning black hole governed by the Kerr metric.

The apparent radius of something residing in a Schwarzschild metric, when viewed from infinity is given by $$ R_{\rm obs} = R \left(1 - \frac{R_s}{R}\right)^{-1/2}\ ,$$ where $R_s$ is the Schwarzschild radius $2GM/c^2$.

This enlargement is due to gravitational lensing and the formula is correct down to the "photon sphere" at $R =1.5 R_s$.

Most of the light in the EHT image comes from the photon sphere. It is therefore observed to come from a radius $$ R_{\rm obs} =\frac{3R_s}{2}\left(1 - \frac{2}{3}\right)^{-1/2} = \frac{\sqrt{27}}{2}R_s\ .$$

There are small (<10%) corrections to this for a spinning black hole governed by the Kerr metric.