Several Earths-worth of air
Cool graphic illustrates how much air Earth has; Virga, it seems, has more
Found on BoingBoing (and previously by them here): a very interesting graphic that displays how much water and air there are on the surface of the Earth. The ball of air appears to be about 2000 kilometers in diameter. Now, in my novels Sun of Suns, Queen of Candesce and Pirate Sun, I posit an enclosed sphere of air more than twice that diameter. I hadn't really thought it through, but this means that my world Virga has several Earths-worth of air in it, probably a dozen or more. So, when I say that Virga is 5000 miles in diameter, that doesn't mean we're talking about a small world, because the entire volume of this sphere is living space, whereas on a planet only the surface is livable. So the ecosystem of Virga is far, far bigger than that of the Earth, or even of the Earth and all the terrestrial planets combined (assuming they were terraformed) by a considerable multiplier.
When I set out to write Sun of Suns, I conservatively estimated about 120 artificial suns and attendant nations inside Virga. I imagined that each nation might have a population in the low millions, but once again if you look at the volume lit by the suns instead of the area of the circle they light, it's probably safe to say that Virga could hold tens of billions of people without overcrowding.
And to think, Virga is a small world by the standards of what's possible.