How long and detailed should be the rejection comment when reviewing and rejecting a paper?
Short answer: I think it is very disrespectful (or not good) to reject a paper by writing one single paragraph. If a reviewer thinks that there are obvious flaws that cannot be described clearly, then the reviewer shall simply not review the paper. 

The reviewer may have not understood the paper well: Personally, many times I get a first negative impression of a paper. I try then to write a summary of the paper. Then I realize that "Oh the author meant this !" or "Oh, this actually works !" -- this usually lead me to realize at the end that the paper is good. 

The authors spent a lot of time writing the paper (they are looking for constructive comments most of the time): From another point of view, put yourself in the authors shoes. They have spent a lot of time (obviously) writing the paper. They are smart people (otherwise, they would have not reached this level to write a paper). Then obviously, there is something to get from them.  If the reviewer thinks that this is not enough, then strong comments should be written. In any case, a reviewer should spend a relatively long period of time in each page of the paper. 

The reviewer may not have treated the problem from all perspectives:
Note also that - in some cases - the reviewer may have experienced the problem from one single perspective, whereas the paper attacks the problem from a different perspective. Treating a problem with a different perspective is the best way to solve the problem IMHO. It only enriches the literature.