I'm honestly not that strict with length, and paragraphs are very subjective.  To answer your question, I guess one meaty paragraph or two or more less-meaty paragraphs are good.  If you are in the mood to write more, then go for it, but I'm not going to expect a minimum of five fully fleshed-out paragraphs (I left essay writing behind me when I got out of grad school).  I usually avoid the 'free' classification because I'm afraid of speedposting (or expectations that I be on all the time) and poor grammar.  I'm fine with one-liners if it's just combat or conversation, but I really just want someone who can be proactive to share the load of moving the plot forward instead of just reacting to what I give them.  Even though I said my post frequency was about one per day, that applies to IC because I sit down and try to put a great deal of thought free of distractions into my IC posts; I post a bit more frequently when discussing things OOC, especially when starting a new RP, because taking forever to brainstorm can cause an RP to get stale before it has even begun.  I see from your avatar and signature that we probably have similar tastes.  Any ideas pique your interest?