Interesting question. Permit me to toss my hat into the ring, if you will, beginning with a piece of my history.
The first forum I ever used for roleplaying was based around a particular indie game studio's work. Its roleplay section, way down at the bottom of the page, was split into three sections, titled as "Role Playing Academy", "Role Playing", and "Advanced Roleplay". For my entire stay there before the forum absolutely imploded under the unresolved sexual tensions of 40% of its population and became too salted to sustain my spoiled oversensitive hide, I was in the former most category; easily the most alive and interesting and... well, fun of the three subcategories. The requirements for being allowed into the upper echelons of the board were nebulous but definitively linked to post length. I, being the naive little sprout I had been, decided to peruse the Advanced section to get a feel of what might be expected from the "greatest of the greats" as it were. What I saw was rather impressive; a small cadre of some six active people typing monolithic posts and wielding ungodly elephantine character sheets containing every subtle detail described in at least seven different variations, stories spanning hundreds and thousands of lines of intricately-woven prose dedicated to squeezing out every microcosm of detail even remotely possible.
To be brief, I read, and I wept. For I was bored to fucking tears.
By conflating an RPers skill, nay their very competence, with word count, they had instilled in the community a belief that the best possible RP to be had was something dry, sterile, and hollow which could be dissected at anyone's inconvenience to be used as a less-efficient thesaurus. And so nobody advanced, not because they weren't capable enough, but because... well, why bother? The fun was at the bottom rung, where everyone else was. Needless to say, I left after the community was divided not solely over bitter feelings, as I had been entirely uninvolved in the debacle (ah, computer troubles, how I despise you sometimes), but mostly because the people leaving were the ones I like hanging around with.
Fast-forward a few years and good number of personal demons later, and you'll find yourself in the here & now. My present average is a floating number between three and four paragraphs, largely because of how I format my posts and how I manipulate the flow of the RP; I rarely go below two as I use my first to react to any relevant details my partner has given me, and my second to move us forward if only a little. Each paragraph averages to about six sentences, though that's only because I play fast and loose with syntactic precision most of the time and leave sentences and thoughts conjoined like a normal person might speak. Point is, I find that the best measuring stick for a person's skill at writing in the context of RP, as with most of you, is "did you do anything meaningful to the RP with this addition, or are you simply wasting time and throwing a fistful of words into the mix to appease some underfed sense of obligation?" Because I have an RP going right now where my partner hardly gives me six lines of text, but I'm enjoying the hell out of it. Certainly more than I would have the ten-thousand word Advanced Roleplay section on my original boards. I also have one where my partner gives me as much as I give them, and our posts are somewhat large but filled with little things and expound on tiny quirks in how our characters think and how the world around them moves, not relevant but intriguing little minutiae that helps give everything that third dimension. And I'm still enjoying the hell out of it.
*Bows* I think that makes my case, however redundant.