I agree with what Broomhandle45 has to say.
Before I came to join this site, I thought up the following: If the content is sufficient, then the length will be sufficient. What I mean is that one should work on creating compelling, interesting, purposeful content before length. You can stretch, and squeeze, and milk your paragraphs for all they are worth, but what you have left is a long-winded, bloated read. Instead focus on making a post that deals with the situation at hand, that reveals what needs to be revealed, and moves that narrative forward in an interesting, or even obvious, direction.
The usage of descriptors are in fact very important, but they should not rule your post. They should set the scene, provide characterization, and give important information. To do this every single post becomes redundant. You can only describe the room so many times, and you reminding the reader that your character is smiling every other sentence insults the reader's memory and/or intelligence. A character that switches emotions from paragraph to paragraph is in a rather manic state, and can be hard to believe. This is where pacing comes into play.
Your pacing will dictate the length of your post, and the flow of the narrative. Your opening posts, in any one scene or new situation, should be among the longer of the collective of posts that you will make during any one role-play. As mentioned above, they will set the scene. You will use these posts to describe the environment, the situation at hand, the motivations of your character, and how they feel about where they are and what they are doing. After this post, you are likely to then change focus. Depending on the reply you receive, you may then focus on your partner's character. Your descriptions are going to focus on that character, and perhaps how your character feels about them. Another post in, and perhaps you have less to describe. You are now only writing about how the character might feel about what is being said, or an action taken by another character. What you are writing now is noticeably different than your opening post (for the scene or the entire role-play). It is perfectly reasonable for these posts to be much shorter in length.
Consider a real conversation. When speaking with someone, generally, it's a quick process; it's a back and forth exchange of information and ideas. Your focus is on what is being said, and what you are going to say in return. There is normally very little time for deep though, and introspection. Granted this may change, if the person I'm speaking to is particularly long winded. Consider the urge to internalize after a single, simple collection of words has been spoken to your character-- a simple sentence, or greeting. Is your character actually likely to think paragraphs worth of content? They'll end up just staring blankly at the person that greeted them, while making odd expressions and running through emotions. Oh, but perhaps the intention isn't to actually write about the character's current consciousness. In that case, why not use characterization to express those thoughts and feelings? Why tell your partner that your character finds their character to be a sex symbol, or the most frightening warrior ever? Instead, use characterization.
When writing, it takes a decent amount of skill to properly express your character by showing versus telling. It is not the easiest thing to do, but it is the most effective. A character cannot be mysterious when their thoughts, plans, and designs are laid out before my eyes. A seductive, sexy character is not sexy or seductive when I am merely told they are. A martial arts master is not a martial arts master, when you "tell" me they beat up the group of thugs hassling my character. You must show that they are what they are. You do this by considering an actor. What does an actor do to convey a character? They use posture, tone of voice, mood, and method of movement. You describe these things, and you do so in a clear, concise manner. If you delve heavily into the details of every action, then you dilute the significance of the important actions. Actions speak more loudly than words, and this is true even in text.
I'd like to mention that a paragraph is not any set length. A paragraph can be a single sentence, twenty, or more. It is only when you properly express your idea that you move onto a new paragraph (interesting literary devices aside). You should not move onto a paragraph only when you hit five, or six, or seven sentences. Length does not make interesting content, otherwise the dictionary would be a riveting read.