Some signature campaigns enforce a minimum character requirement. That just means that posts which have fewer characters than the limit won't be counted. I know Bitmixer.io has a 75 character limit and so does mine (Coinomat) but I'm not aware of any others that have one. Yours (777coin) for example doesn't.
It's possible that a short post can be constructive. And while perhaps a bit more rare, it's also possible for a longer post to be unconstructive. But generally speaking, unconstructive posts tend to be shorter in length than constructive posts and enforcing a minimum character requirement tends to improve the quality of posts.
and for example i am a newbie and i havn't any experience in the bictoin and this that i try learing about it
so i havn't any information how i can write a constructive post