Fair, I have no idea if it confirms the size, though maybe the specific speed of the gas helps with that. Mostly it's evidence that there is a black hole, but it's not supermassive, and a stellar-mass one probably wouldn't have enough effect to be noticed.

You can determine the mass of something by measuring the time it takes for an orbit around it, yes.

You need the orbital radius, too.

Yep. You can get both from observations of the orbiting object.