Text roleplay is different from writing and reading stories. When reading novels, stories, news/reports/other factual sources of information, or poems, it is (in most cases) a one-sided deal. The authors have produced a story that will be read the same way each time. The impact it has on the reader will change from person to person and can be re-read for a new perspective and interpretation multiple times, but the reader has no impact on how the plot progresses (except for CYA books, obviously). Roleplaying, on the other hand, is a collaborative effort to tell a story or play out scenes without a fixed plot. Even in instances where the story has been detailed beforehand, the amount of uncertainty in aspects like characters' responses to situations makes roleplaying more dependent the interactions between writers and characters.
Unless you're playing out some very banal slice of life meandering between characters, interactions typically follow a loose formula where one character leads the action or dialogue, and others respond to the action or dialogue. We can call these the acting partner and the reacting partner. This extends to a lot of media and permeates most of roleplaying, and the acting and reacting roles may switch depending on the scene. Sex is obvious. If one character is dominating or leading penetration, their input (or lack of it) is necessary for the other writer to make their character respond in kind so things can move forward. Action scenes follow the same process. In dialogue, the acting partner asking questions or giving orders and information gives room for the reacting partner to respond. Whether each writer and character is put in the role of an acting or reacting partner is left up to a few factors, including authors' choice (consider that the majority of sexual dom/sub roles aren't determined in-scene, but by the writers seeking compatible partners) and circumstances of the scene.
How does this relate to post lengths? Consider a few things commonly accomplished in a post: introducing a writer or character to somebody new; greeting a familiar partner; dialogue; scene-setting; action. Everybody's roleplay experience and preferences are different, but I've seen that the majority of roleplays I've engaged with do multiple of these things per post. A shorter post can only have so many actions and description within, and longer posts have more room to play around with wording to get the points across. Somebody's choice to prefer or even require a certain post length is their own, and there's no sense in judging them for it in hopes they'll concede to your preference. Here are a few strengths and weaknesses of some post lengths I've found to be popular.
Short (3 or less sentences)Obviously, this is an imperfect and incomplete list of pros and cons for the limited categories I chose, and there's clear bias. Still, it paints the clear image of where different post lengths may shine and where they'd be unhelpful for the goal. Large word counts are sometimes used as a mark of somebody's roleplaying proficiency, as if larger posts automatically improve the quality of their writing (there's also a noticeable undercurrent of elitism on F-List concerning post length, word count and post times among other things, but complaining is unproductive so I'll try to stay impartial). Instead of typing text to fill space for the sake of it, instead consider how well you move the scene forward. This applies for posts that are too long and too short.
Lets return to what I mentioned earlier about the acting and reacting dynamic. Acting and reacting can take a few forms. The most obvious form of action is... performing an action. Initiating conversation, asking a question, pointing out or describing detail, fighting, striking, fucking, thrusting. These are all details that the reacting partner needs to account for in some way unless it's only being mentioned for minor flavor or to fill up space. Lack of action or detail gives the reacting partner nothing to go off of, and too much restricts them into needing to respond to each point.
Additionally consider that not every post will work with a specific number of sentences. If one writer gives a large, detailed paragraph and the other writer's character only gives a simple response, there is a clear discrepancy between how much effort each person has put forth. Even if a simple response is warranted, the extra space should be filled somehow. This is not to say sentence counts should be matched exactly, but it isn't a bad goal to be had. The same can be said of the opposite situation. Except in fringe cases, a simple action shouldn't need a response that's significantly longer than what it's reacting to. Even ignoring the acting/reacting dynamic, you can consider how a response that's too short or too long can make it difficult to move the scene forward. Both have the capability to provide no progression.
All of this reads like thinly-veiled frustration with roleplayers that write too little or too much, but I promise that it isn't. Instead, it's fully unveiled frustration with writers that have no desire to be flexible with their self-ascribed post lengths. I believe the optimal solution is flexible post lengths, which fill in the gaps mentioned earlier
FIRST I set the glass down on the table and slid it towards you. Narrowing my eyes, I cleared my throat to catch your attention and spoke up. "Well, (your name)? What do you think?"
THIRD A set the glass down on the table and slid it towards B. Narrowing their eyes, A cleared their throat to catch B's attention and spoke up. "Well, B? What do you think?"
SECOND A set the glass down on the table and slid it towards you. Narrowing their eyes, A cleared their throat to catch your attention and spoke up. "Well, (your name)? What do you think?"