You cannot use underscore (_) in domains where as a hyphen (-) is allowed but I would not recomend that either as it should be something which members can easily remember. As far as length is concerned I would not recomend going above says 12 characters again the reason is same you want something people can remember. Domains only have about 10 percent of effect on your SEO, its better to have your primary keyword in the domain but I have seen sites getting bit without it.

The only effect to having hyphens is it separates out keywords in your domain name. For example, the URL www.computerforum.com means absolutely nothing to Google. It's just a long string of characters, since there is no recognizable word "computerforum"

However, the URL www.computer-forum.com will boost your ranking for the keyword "computer" and also for the keyword "forum"