Forum posts can very in length depending on the subject of discussion. For instance, a thread like " what's your favourite food?" Require a simple answer in the length of a sentence. Some topics need in-depth discussion.
Personally, I think one paragraph is adequate for a good lengthy reply post. However, if you're starting a new topic, I'd say shoot for at least three good paragraphs, an introduction, some supporting meat, and a conclusion (question). Each paragraphs should probably average about 45 words long. (But who's counting, right?)
If all you're doing is replying with a single sentence, you're really doing a disservice to the community as a whole, because sure, you can simply state your opinion on the topic, but a single sentence has nothing to support your take on something. More words gives people something to latch on to.
As a side note, this is especially true with Play by Post role playing. I see on various forums where players and game masters alike get frustrated by the one liners. There's not much that can responded with from a single sentence or worse, single word reply or the dreaded silence reply "...". One thing I now try to do when role playing is attach an Action Beat to any dialog response that my character is doing. This allows for other players to latch onto my post and do something. This same philosophy holds true with regular forum posting as well, in my opinion. People need something to grab on to.
One word posts are better reflected in emoticons. Hopefully with words too. But I don't feel comfortable requiring minimums (in my niche) as I too have been known to do that myself.
I've thought about mentioning a minimum "good quality" word count for a post, but it's never going to be a requirement. More like a guideline. Something for people to reach for, and maybe a mechanic put in place that rewards good behavior like that. In the past, I've had people on my forum who would make a post shaming members for sharing low quality posts, and I thought about ways to reduce this type of shaming behavior. Something passive in the background is much more preferred than out right ridiculing somebody for contributing a lazy post. The first time may generate a chuckle, but it gets old really quick, and can become a headache for administrators / moderators if they have to babysit members posting behaviors. It's just not a needed headache.