I am doing this totally from memory so they numbers may be off. But Mars is like 1.52 AU’s from the Sun and Jupiter is around 5.2 AU from the Sun. (These are the average AU’s) That means, at their closest, Mars and Jupiter are about 3.5 AU’s.

I believe you can fit around 10 or 11 Jupiter’s across the diameter of the Sun.

If Jupiter was the same size as the Sun, it would be just under a third of the size in Mars sky at their closest. But Jupiter is roughly a tenth of the size of the sun, so it would be a tenth of that size.

I think I am going a little large on the size, but the two planets would get closer than their average AU’s would indicate, so I think these two cancel each other out.

I drew it out on a piece of paper. I started out with the sun about the size of a quarter. I drew a smaller circle with a third of the diameter. Inside that one I drew a smaller circle with a tenth of a diameter of that circle.

I think the disk of Jupiter could be seen from the surface of Mars at their closest.

Very rough calculations.

In relation to a full Moon as seen on Earth, I wonder how far from Jupiter a moon would get as seen from Mars during their closest approach?

I am thinking just under a half of a full Moon’s diameter.