If I have no idea how to describe something or what it might look like, I can crowdsource the answer.
Below is the question I posed to r/askscience on www.reddit.com/r/askscience
I'm writing a novel that takes place on a large (fictional) asteroid in orbit just outside (but still within the boundaries of) the Asteroid belt. My real question for the sake of description is how large would Jupiter look from this perspective? - assuming that Jupiter is at the perigee and aligned with my fictional asteroid's orbit. Would you be able to make out the Red Spot or any of the moons with the naked eye? Would it be the size of the moon? Larger? Smaller?
I posted this in  /r/astronomy and someone was very helpful in pointing out that Trig is a good way to find this sort of thing, but I'm a writer and haven't the foggiest idea where to start with that.
And the answer quickly sent back for me:
I think this sort of thing is generally measured in  minutes of arc.
Now, I've probably messed up my math (read: it wouldn't hurt to do this math for yourself), but if you put your asteroid at about 4.2 AU and Jupiter at perihelion (4.9 AU), Jupiter will have an angular diameter of about 4.6 arc minutes. At the same distance (0.7 AU from you to Jupiter), the Red Spot has an angular diameter of 1.3 arc minutes (taking its true diameter to be approximately 40000 km -- note I took the maximum value that Wikipedia offered).
In comparison, the moon has an angular diameter of about 34.1 arc minutes at perigee. The star (other than the Sun) with largest angular diameter has a diameter of about 0.05 arc seconds (although this number gets somewhat inflated due to atmosphere when viewing from Earth's surface).
Using midnightbaconz' number of about 1.2 AU puts Jupiter's diameter at 2.7 arc minutes, and the red spot's diameter at 0.76 arc minutes.
To give you an idea what minutes of arc mean for the naked eye: the 'nominal' 20/20 vision row on an eye chart has letters which subtend 5 arc minutes (each). As I said, I've probably messed up the math, so I will leave out my conclusions and let you draw your own.
It's awesome what you can do with the internet these days as a writer. Research has become infinitely easier.