Here is what I think is a constructive post:
1) To the point answering the query asked. Reality is 905 people here just post fluff (trying to add as many words as possible even if it makes no sense).
2) Posting on new topics and not answering questions with numerous relies until you have something new to add.
Does the character count attribute to upgrading your status?
No. This has been answered here:
https://bitcointalk.org/index.php?topic=2766177.0Though short posts are probably less likely to receive merit than longer ones which you now need to rank up. People just used to 'fluff' their posts and stretch them out because they thought two sentences magically makes their posts constructive and look like they put some effort in. It doesn't. The people who do these two liners are some of the worst posters here because they just ramble on adding needless words to something that could have been said in far less. Though, as LeGaulois said above, sometimes a response only requires a word but a constructive poster would have a varied post history ranging from one word posts to paragraphs and everything in between so don't just try make your posts hit some kind of character count just for the sake of it as it usually has the opposite effect.