My advice is: just write what feels natural.
One thing I've noticed -- especially after I've been RPing with someone for long enough to be familiar with their usual posting style -- is that it's really easy to tell when a post was either padded out for length, or stripped down to the bare essentials so that it could be rushed out as quickly as possible.
In the case of the former, I often see sentences that really look like they were added just for the sake of length. They could easily be cut out without the rest of the post suffering for it (and usually look like they were added after the post was already 'finished'), and often just seem like they're just grasping at whatever details they can to just pad the post out a bit. Now, that's not to say that every detail-filled post suffers from this. There are plenty of long, detailed posts where every sentence looks like it really does have a purpose (in fact, I often find that these I'm-just-giving-this-more-sentences-so-it's-longer posts are usually shorter than the it's-long-because-there's-a-lot-I-want-to-say posts, because the former usually only happens when someone feels like they need, say, at least one solid paragraph in order for their post to feel long enough, so they feel like they have to pad out what would otherwise be a short but serviceable post). I find that when RPers are actually feeling inspired to write a long post for whatever reason, it works a lot better. Like, maybe you've really been looking forward to this scene and you've let it play out in your head like a movie so many times that you really want to capture every detail from your mental image; maybe you really want to get in all those details because they'll really capture the atmosphere of the scene; maybe you're trying to include some symbolism that relates to what your character is going through; or maybe you're just feeling artsy, and you want to describe things in pretty ways because you thought of artsy ways to say things and you want to make use of them. Whatever the case, it's usually pretty easy to tell whether this person added this level of detail because they wanted to and because they really invested themselves in it, or whether they put all that in because they felt like they needed it and wanted to find some way of making their paragraphs look meatier.
As for posts that seem shorter than usual -- like I said, they usually just look rushed (which is unsurprising, given the fact that they usually come from players that I've been waiting on for a while). That's not to say that every short post is this way -- just the ones coming from people who usually write a lot more. Additionally, these are posts where it usually seems like there are a lot of things going on in the scene that the character should be reacting to, but the RPer just gets straight to the point and just tells us in one sentence or two what exact action or line of dialogue their own character is adding to the table. I don't mind short posts in principle, but... if you're rushing it, then, these posts just feel so... empty. I'd rather wait a bit longer and get a quality post than have someone rush out a post when they're not feeling it and have that long wait be met with... well, a post that didn't at all feel worth it. :/
So, in general, I just like it when RPers write whatever feels right to them. Don't pad it out to "match" my post length, don't rush to get it out quickly -- just sit down and write whatever you think flows best. This is especially true for the padded out posts, as I see them most often from players who think that they need to be writing more, and it's kind of saddening to see players fill their posts with clunky and unneeded sentences just to make their paragraphs bigger. Not only are you spending time and effort on something completely unnecessary, but your posts would actually have been better beforehand. :/
Besides, if you write based on emotion and inspiration as opposed to trying to meet some arbitrary length every time, then the long posts are a lot more meaningful. Then, long posts will be as detail-filled as they are because you're putting a lot of investment and emotion to it -- which really shows in the case of, say, emotional scenes that you were really looking forward to writing. There's just so much more soul in it than a post that's only long because it was padded out. It makes these posts much more of a joy to read, and it makes me much more eager when I see a long (or at least, longer than usual) post, because I know I'm in for something good. ^^