#tumblr absolutely has a maximum post length #or at least had one #and I hit it#I want to say it's 10-15k range? #those theory posts went hard (via @ariabauer)
From a technical standpoint, Tumblr's post length limit is either one megabyte of raw plaintext or 1000 "content blocks", whichever comes first. In this context, a "content block" means a heading, paragraph, a bulleted or ordered list entry, or an embedded resource like an image or a video. (Embedded resources are not counted against the one megabyte raw text limit, but have their own individual limits).
It's difficult to imagine that you managed to hit one megabyte of raw text in a single theory post (for reference, that works out to roughly 180 000 words, assuming you're writing in English, or about six hundred pages of formatted text), so I'm guessing you hit the content block limit instead, probably through some combination of numerous very short paragraphs and going hard on the bulleted lists.