I have forgotten the exact size limit per post but I think it is 64kb but some symbols are much larger, and new lines may be larger so if you keep each part down to less than 50kb ish maybe it will fit?
Also it depends on how you want to do it, you could make a link at the end of each post to the next one, I did that for a story of mine that is not around here anymore, or you can make the first post an index with links to each part/chapter of your story, or you can reserve a bunch of posts in a thread first, it is mostly up to you, also if you do make links try to do it to the https:// site, that way everyone will stay logged in when they click the links, and be able to comment after reading.
Also please read the rules for what stories can be posted and properly add tags to the title of your thread.