I think domain names can be of any size, as long as you can brand them and they are easy to remember. For instance, WHDF.com is very easy to remember, except it has 4 different letters which some may find difficult to keep track of. Yet, if you had WebHostingDiscussionForum.com, I would find that much easy to remember. It has 4 words, and while it may be a pain to type in every time, it's very brandable and can be put to good use.