If Jupiter was placed in Mars' orbit, how would Jupiter appear from the naked eye? If Jupiter were as far from us as Mars, how big would it appear in Earth's sky? Would it appear huge, or would it be as small as a star?
Since the question is actually asking for a size of Jupiter as seen from Mars, there is a very simple equation that will give you a good idea.

$$\frac{\alpha}{360^{\circ}}=\frac{\ell}{2\pi r} $$
$\alpha$ is the angle that Jupiter would subtend in the sky, or the angular size, and $\ell$ is the actual diameter, or linear size, of Jupiter. $2\pi r$ is the circumference of Jupiter's orbit about Mars. Now I know what you're thinking, that this doesn't make any sense because of the vast differences of Jupiter's orbit when it is close compared to when it is at opposition. It'll work out...

The following are all approximations.

Mars orbits the Sun at $\sim1.53AU$ and Jupiter at $\sim5.2AU$. Because $1AU \approx 150Mkm$, The difference in their orbits are roughly $550.5Mkm$. 

$$\alpha=\frac{140000km}{2\pi(550.5\times10^6km)}\times360^{\circ}=0.015^{\circ} $$

That is just the average angular size, at the average closest distance, but given that the only real tool to measure angular size with the naked eye (that isn't a sextant) is your hand, and your smallest digit (the pinky) at arms length marks out $1^{\circ}$ you'll never be able to tell the difference in the variation of size due to the change in distance.

Now answering your other questions directly:


Jupiter would look larger, but not enough to actually tell a difference in size.
As shown above, Jupiter would be much smaller than the moon appears to us from Earth, $0.5^{\circ}$.
No disk, as this is very much a point-like size as far as human eyes are concerned.