Oh lol. I once saw a post here of a guy who made a long post and other squids said that it made their eyes hurt etc. They didn't even read it lmao.
I do agree posts can be too long, but a lot of it has to do with the structure, efficiency and quality of the writing as much the length of the post itself. If a post is hard to follow, fails to get to the point or is overly repetitious, then I will roll my eyes and move on. But if a post is trying to communicate a very complicated point but is constructed in an understandable and efficient (or at least entertaining) manner, I'll keep reading.
Obviously it's also has to do with whether the subject of the post is of any interest to the audience. Folks might read it if it's short (and then probably just ignore it). If it's long they almost certainly won't read it, but that's more a function of people spending their time on things that are actually of interest to them than anything else.
The thing is, the only way to become better at writing is to do it. And maybe you'll write a clunker of a post (or three) but it will be a learning experience. I kind of think as long they aren't being offensive and are generally on topic people should write what they want, though they should probably try to keep in mind what the experience will be like for anyone reading what they put down. At least if they want to be heard.
Edit: I guess I should add, I do think it makes sense to try communicate ideas as efficiently as possible (which is not what I'm doing here lol). That generally leads shorter, more digestible posts, but does not constrain one from writing about complicated ideas.