By a straightforward bit of trigonometry, if the distance from the observer to the (centre of) the planet is x km, then the radius, r km, of the planet subtends arcsin(r/x), and so the angular size of the planet is twice this: 2arcsin(r/x)
For Io x = 420,000 km, and the radius of Jupiter is r = 70,000 km, so the angular size is
2arcsin(1/6)=19circ. For comparison, the moon from the Earth has an angular size of about half a degree.
Stellarium can be set up to view from other worlds of the solar system
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