Do all authors contribute to a reviewer response or only the main authors? I would like to know if all authors contribute to the reviewer response document for first revision, or do only the main authors contribute?
The authors can arrange this however they want. If they all want to work on it actively, that's fine. If they'd rather agree to have one person write the response and then send it to the other authors for approval, that's fine too. All that matters is that everyone approves. Unlike the paper itself, it's not necessary that all authors make a significant contribution.

It's impossible to give any general advice on length: in some cases a short response is sufficient, in other cases you might have to write more to express what you need to say. But conciseness is generally desirable; nobody likes to read unnecessary text that doesn't add anything.