I think many Warriors are avoiding this thread because it is based on a false premise that seems to flow from a fundamental misunderstanding of the term “long tail“.
First things first, the term “long tail”, when properly used, does not refer to the length of a keyword term. It refers to the frequency of occurrence within the distribution of a data set. If it occurs less frequently than the most commonly occurring keywords within a data set and that data set includes a relatively large number of keywords that also occur with a lower frequency than the most common (short head) keywords then it is considered a “long tail” keyword. it could be a single 3 letter word or a lengthy 5-word term, the number of words or characters within the term has no bearing on whether it is a “long tail” term or not.
Once you understand the term “long tail”, it makes the OP’s question self-explanatory. If it is a long tail it means it has little to no search volume. So it will also have little to no value as a domain name.
The best domain names, from an exact match keyword perspective, are domain names that have high relevance and are among the short head, not the long tail. Everything else being equal, the shorter the domain name the more valuable the name.
Here are the primary domain name factors that you need to consider when selecting a new domain name (in order of importance):
- Easy to remember
- Easy to spell
Those 3 primary factors should guide your domain name selection above all other factors.