How to calculate a planet's apparent size when the planet is viewed from a moon in orbit around it?

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
$2arcsin(1/6)=19^circ$. 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