What is the actual resolution of Event Horizon Telescope?
The resolution of an interferometer at a wavelength $\lambda$ is $\theta\sim\lambda/b$, where $b$ is the longest baseline (distance between two telescopes) in the array. According to the first of this week's six papers, with observations at $\lambda=1.3\text{ mm}$, the longest baseline was $b\approx10700\text{ km}$, giving us $\theta\sim25.1\mu\text{as}$. The team expects this limit to be lowered even further with the addition of more observatories.

The picture, of course, seems much finer. The above limit can be lowered a bit by algorithms like CLEAN or regularized maximum likelihood; this brings the resolution down to about $20\mu\text{as}$. This still doesn't explain why the image seems even finer than that figure; that comes from image synthesis techniques that smooth out the data, rather than leaving it in a coarser form.