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.