Given I can't see anything at first glance about needing a specific word count before posting a story, I'm going to hazard a guess that it's because the forum is going to be upgraded this week. I may be wrong, however, so if anyone knows then please correct me.
As for formatting a multi-chapter story, there aren't really any set-in-stone rules. But I can recommend a good word-count per chapter (I tend to stick to around 3000 words minimum, although I will go way past that if the story calls for it. I know a lot of people write much more than 3000 words per chapter.) Each individual post would be a chapter, really, but sometimes bigger chapters would need two posts. Some writers, myself included, like to link the chapters in the opening post for easier navigation on chaptered 'fics.
As for structuring the story itself:
If you're a planner, I'm not really the person to advise on that. Someone who plans extensively may come along to advise here. But programs such as Scrivener can help you to outline a story. I use Scrivener and it's fantastic (and in some ways encourages me to plan out a little more than I would usually.) You may also be able to find some outlines online or in writing books. 'Ready, Set, Novel!' for example. Although books like that are more tailored to help you plan the novel itself all the way down to its characters and world building. Am I right to assume you are past that stage?
For multi chapter stories, you can build around arcs or just go with the flow. Arcs are a lot of fun, as you can sort of theme it like an anime series and give yourself time to work out and build on big events. For a pantser like myself, I love this format. I can go wild with my imagination and off on a tangent without worrying about deviating away from the plot too much because after one arc is over, it's right back into the thick of the main plot like a text-based boomerang.
One line of advise I can really give when it comes to chapters is to end when the chapter feels like it should end. I did give a vague guide-line for a meaty chapter, but that's not to say chapters can't be shorter. (Alice Through the Looking Glass had a chapter that was simply one line. 'And it was a kitten after all.' So there is no real rule to how long a chapter should be.) It's a good idea to open with a hook and end on a cliff hanger. But don't abuse the cliff hangers. You do want your readers to keep coming back whenever you update, but constant cliff hangers might make it seem a bit too dramatic. You'll want to end on a positive note at times, too.
I can also recommend getting into an update schedule. I pick a day a week and stick with it. That way, a routine will keep them chapters coming
Although atm I'm on hiatus until April due to other commitments. If you do need to take a hiatus, make sure readers know so they don't think the story is abandoned! Chaptered 'fics take time and short breaks are almost always inevitable! A way around this is to give yourself a chapter buffer of say around 10 chapters or so, so you can keep updating when you need to take a break from writing. This also allows you to make changes if you need to. The draw-back of arcs here are that if you do have a buffer, you may also want to have completed the full arc to make sure the story flows without continuity errors. Although that can be avoided with planning.
I hope this helps a little =)
