How big would Callisto appear from Ganymede?

Ganymede 5,290km 1,070,412km
Callisto 4,821km 1,820,000km

Callisto is about 1 1/2 times the size of our moon, and at closest approach, is about 790,000 km from Ganymede. So, it would appear slightly smaller than our moon.

The easiest way to get this is with any good planetarium program, which will let you go to one moon and let you observe the other and get data on it. The size will of course vary as the two moons move around Jupiter.

Callisto would appear about 5.6 to 20.7 arc-minutes in diameter. For comparison, our Moon is about 31.7 arc-minutes in diameter.