Because I have no life, I decided to do some hard soft math on the problem. I used Orbits 3.0 (a rather old orrery program), the "Jupiter Fact Sheet," and the famous "Solar System Simulator." to arrive at these answers.<br /><br />The fact sheet tells us that Jupiter's largest apparent size is 49 arcseconds, and that it's maximum brightness is -2.94. I chose two dates to examine--one when Jupiter was near opposition with Mars, and at its perihelion (May 1, 2011), and the other when Jupiter was at aphelion and near opposition with respect to Mars (February 21, 2018).<br /><br />First of all, the advantage of Mars's greater distance from the Sun is not really capitalized on, since Mars and Jupiter reach perihelion and aphelion at nearly the same points in their orbits. <br /><br />In any event, on May 1 2011, Jupiter will show a disc of 55.14 arcseconds from Mars. This is only a little more than 6 arcseconds wider than it appears from Earth, though the planet will also be some 27% brighter, nudging it up to around magnitude -3.2. <br /><br />On February 21, 2018, Jupiter will subtend 51.22 seconds of arc--larger than it ever gets from Earth, but not by much--and appear only 9% brighter than it does from Earth at its best, or around magnitude -3.03.<br /><br />