Length of post isn't really relevant when it comes to deciding whether or not a post contributes to the discussion/if it is useful, or reporters/moderators deciding if a post should be reported or deleted. A post of a single word might be perfectly acceptable if it is answering a direction question, and conversely there are some users who post entire pages of text which are rambling, incoherent, and not worth reading. Indeed, if you are writing a post and the whole time thinking "How can I make this post longer?" as opposed to "How can I get my point across?", chances are you will end up rambling, going off topic, or repeating yourself, all of which makes your post more likely to be deleted, not less. Try to say what you need to say as succinctly as possible.

What's more important than post length is being on topic, saying something which contributes to the discussion, and not just repeating or rehashing what other people have already said (so read an entire thread, don't just immediately respond to the first post).