I know they can "see" the ball of gas it's emitting, but when they specifically mention the size, it does matter.
They are not detecting the size directly, it’s based on assumptions about the type of object it is and calculations about how the gravitational forces and nuclear forces of such an object would interact to determine its geometry.
Basically, if they can figure out how much mass it has and what type of star it is, they can guestimate the diameter easily.
Well, the size is obviously calculated from observation data. When I said the size of the source doesn’t matter, I was thinking along the lines of seeing a large bonfire from 10 miles away vs. seeing the light emitted from a tiny LED. You see the light, not the object.