Domain names can be of any length up to 67 characters. You don't have to settle for an obscure domain name like avab.com when what you mean is AcmeVideosAndBooks.com.
Having said that, there appears to be some disagreement about whether a long or short domain name is better.
Some argue that shorter domain names are easier to remember, easier to type and far less susceptible to mistakes: for example, "getit.com" is easier to remember and less prone to typos than "connecttomywebsiteandobtainit.com".
Others argue that a longer domain name is usually easier on the human memory - for example, "gaepw.com" is a sequence of unrelated letters that is difficult to remember and type correctly, whereas if we expand it to its long form, "GetAnEconomicallyPricedWebsite.com", we are more likely to remember the domain name.
Some of these arguments are actually academic. It's increasingly difficult to get short meaningful domain names. I have not checked, but I'm fairly certain that names like "getit.com" and "good.com" have long been sold. If you manage to get a short domain name though, the key is to make sure it's a meaningful combination of characters and not the obscure "gaepw.com" in my contrived example above.
Long domain names that have your site keywords in them also have an advantage in that they fare better in a number of search engines. The latter give preference to keywords that are also found in your domain names. So, for example, if you have a site on free C compilers with a domain name like freecpluspluscompilers.com, it might fare better in a search for "free C compilers" than my other site, thefreecountry.com.
Which would I go for? I'd go for the shorter name if I can get a meaningful one, but I'm not averse to longer names. However, I would probably avoid extremely long names verging on 67 characters. Aside from the obvious problem that people might not be able to remember such a long name, it would also be a chore typing it and trying to fit it as a title on your web page.
