Science fiction writers and scientists have imagined the possibility of exomoons orbiting giant exoplanets in other star systems being habitable.
And there have been scientific articles discussing the possible limits of exomoon habitabilty.
In my answer to this question What do I bear in mind, creating an earth-like world with TWO moons? the last part of my answer links to this scientific article discussing exomoon habitabilty:
The author of that article calculates the inner and outer limits, in terms of radii of their planets, for the orbits of habitable exomoons.
And from that range of orbital distances I roughly calculated that a habitable exomoon would orbit at distances such that the giant exoplanet that it orbits would appear to be between 5.7295 to 22.9183 degrees wide, about 11 to 45 times the angular diameter of the Moon.
I may also point out that among those moonsof a giant plaent which are large and massive enough to be gravitationally rounded, the smallest moon with a rounded form would appear on the borderline between a dot of light and a disc with a visible diameter when it was about 5,729,582.7 kilometers, or 3,560,197.6 miles from another moon. So whenever a smallest possible rounded moon was closer than about 5,729,582.7 kilometers, or 3,560,197.6 miles, it would definately appear as a disc. And if a moon was larger than the smallest possible rounded moon, it would appear as disc in the sky of another moon even if the distance between them was more than about 5,729,582.7 kilometers, or 3,560,197.6 miles.
The two moons that are closest to each other are Io and Europa. They seem to be about 250,000 kilometers away from each other. Would it be possible to see one from the other with the naked eye if they were aligned?
because Io and Europa orbit Jupiter in different orbits, the distance between them is not constant. Instead it is always changing.
The orbit of Io has a semi-major axis or radius of 421,700 kilometers, Europa's orbit has a radius of 671,034 kilometers, Ganymede's 1,070,412 kilometers, and Callisto's 1,882,709 kilometers.
So the distance between Io and Europa varies from about 249,000 to 1,092,734 kilometers. When they are farthest apart they are on opposite sides of Jupiter and hidden by it as well.
Europa, with a diameter of 3,121.6 kilometers, is the smallest of the Galilean moons. At its farthest from Callisto it is about 2,553,743 kilometers from Callisto, and according to my rough calculations should appear to be about 4.2 arc minutes wide from Callisto, about an eighth of the diameter of the Moon as seen from Earth. Of course Europa would be on the far side of Jupiter and hidden from Callisto anyway.
Io has a diameter of about 3,650 kilometers, and can be as close as about 249,000 kilometers to Europa. According to my rough calculations Io can appear as wide as 50 minutes wide in the sky of Europa, about one and two thirds the apparent width of the Moon as seen from Earth. Ganymede should appear approximately that wide from Europa when they are closest.
So all of the Galilean moons should appear as discs in the skies of the other Galilean moons whenever they are visible from them.