15,000 words? Come on, guys, please. 

Realize that 1000 words in a post is about two-three pages in Word (give or take, it depends on your word usage). 15,000 words? We're talking about a 30-40 page post here. 

Let's be real. NOBODY WRITES 15,000 WORD ROLEPLAYING POSTS. The [i]longest[/i] post I have ever seen written in all my years in roleplaying (I've been doing this since 2003) was about 7,000 words for a solo venture that someone was doing. It was barely RPing at that point, it was just someone writing in the same setting but on a totally different tangent. 

If you see a 15,000 word post, it's going to be clear that someone isn't contributing to the story. They're not interested in collaborating, only in showing off how many words they can write. These people need to remove themselves from roleplays and write fan fiction, short stories or actual novels. A reasonable GM, too, should be able to police excessively long posts like these, they don't help but rather hinder the storyline and make it difficult for everyone to respond. 

I don't think you'll find any monstrous posts here at RPG. I would guess the biggest solo posts are going to top out around 1500 words on average, with a few exceptions above that. Collaborative/joint posts may exceed that, but it shouldn't be too much more than double that size. We're still only 1/5th of your mythical 15,000 words right now, so I think you're fine. 1500 and 3000 words posts don't hurt the storyline, they can tire some readers, but then it becomes up to the writers to make the post worth its content. 

To the OP, I think you're psyching yourself out of this. Stop for a moment, step back and take a deep breath. You can't go into an RP being scared of what it might become, if you do, you'll most likely find what —failure— you're looking for. Stay positive. Join an RP that suits your fancy and looks promising. If it doesn't look promising, don't join it, wait a few weeks to see if it gets off the ground. Once you're in the RP, just cruise and let things happen. 

And you know what? Sometimes things fail. Sometimes they don't. But it's like the old saying, it's better to have loved (or roleplayed) and lost than to have never loved at all.