I understand that there may be incompatible measures of length, especially when Latex and open space are involved. I have sometimes posted jpegs, I think those should be counted as fairly short because the effort was not mine. This one is a pretty good candidate, and we should give credit where credit is due:
EDDDITTT: Now that I have looked at a few long ones: I suppose we cannot really measure the amount of time spent by the person answering. However, I can say that Raymond's answers score big in this department: how many seconds does it take the Latex to render, from the instant you open the question?
EEEDDIIITTTTT: Yes, I think that is the way to go. Joriki's top-of-the-charts sits there for about 15 seconds, but then suddenly renders it all. Raymond's new one gets started sooner but takes about 35 seconds to make it all the way through, as I scroll down I can see the new bits going from pre-Latex (gray) to rendered, so I know that the whole thing has not yet been finished.
EEDDIITTEEDDIITT Monday December 31: The beginnings of an explanation. My two very long posts are mostly some C++ output I pasted in. I actually remember re-running the program because the answer page told me 30,000 characters was the maximum. Here is the "trick," such as it is. Raymond's new answer, currently fourth in the ranking, has dozens to hundreds of separate Latex items. The peculiar thing about Joriki's top post is that it is just four separate Latex array items, each very, very long, and overall quite repetitive. So the system probably expects the number of separate Latex items to be comparable to the total mass of Latex characters, and, in short, allows extra character length if there are very few dollar signs.
EVEN MORE EDIT: I copied Joriki's answer and posted it as an answer to For which primes $p$ does $x^2-31y^2=-p$ have solutions? and the system accepted it just fine. I guess I will delete it in a bit, after a few have seen it. Evidently it would take days before Asaf's long-post-search query would notice it....I did delete it, then edited the source down to one line of gibberish. The odd final outcome is that it will not let me add another answer at the same question. So perhaps you can get away with one long answer this way, but it won't permit a second.