I used that calculator in the link. Assuming the size and distance of Ganymede is correct, that is a separate issue, I put in 3270 mile object size and a 390.4 million mile distance, and got a 0.00048 degree perceived size, which when plugged into the same calculator at much closer distances, to better understand the scale of what we're looking at, shows that Ganymede may be just barely visible to the naked eye under the best of conditions, if only it wasn't so close to Jupiter. But it is definitely visible at 16x magnification.
You're problem when using the calculator is you units, Lad. You count in miles, round to the nearest mile (zero) and therefore nothing can be seen. Yet, we can distinguish dimensions that are a fraction of a millimeter across depending on it's distance from us.
As far as starts go, which I wasn't asking about, they seem to never appear as anything even in a telescope, because they are too far. We only see a spec of light, but not a clear image of the source. The details you leave out about the Hubble looking at things billions of light years away are the sheer size of stars and galaxies, and the number of photons they give off in any one direction, and the fact that they will leave the shutter open on Hubble for hours if not days to collect just a few hundred photons over an area much larger than your eye, focused down to a small sensor.