Quality should always be a primary concern. Much can be conveyed in even a handful of sentences with enough flow and context. As long as points are addressed, the categories ultimately fall onto expected capacity.
Free: Some people have the time to knock a dozen posts out per day, regardless of length. With some threads that break my only rule, it might not even matter and may just serve the purpose as a supplementary 'post rate farming ground' for those less inclined to set a scene so much as invest themselves in what their character is doing in the immediate tense.
I've never participated in this site's Free RP and have no right to talk about it, though I respect any measure of investment in the art. We all started somewhere and I'd be lying to say I oozed from the woodworks with wholly comprehensive elaboration and cohesive grammatical enunciation.
I started here back when Quizilla's RP forums were a thing.
Casual: The trenches.
Advanced: Some people are allowed the time to knock out books. 2-5 pages in a word doc, meticulously looked over with spell checks and oral re-readings to pick apart loose ends to form 'perfection'. I've never participated in this style since the length could just as easily be supplemented with collaboration to give communication a more real feel to it.
Idk how it works. Its like speculating what goes on in a nebula based on what you think should be going on.
To each their own.