First thing to think about is the Headline. First aim for a copy is to catch readers attention… and the headline starts the race…and it must include the ‘promise’ for the readers.
It is generally said the brevity helps, because brevity means also easiness, concision, so writers can be better understood by readers. But there are some studies that state the exact contrary. Yes, easy and brief headlines can not always attract many people, because they aren’t so able to explain as the long titles normally do.
Then there is the body. Obviously, it has to be consistent with what you convey in the headline. And it works as the headline. I mean, they have the same importance, because if you launch a promise to draw readers’ attention, then you also have to make them satisfied with your writing.
And here you have to decide whether to use a long or a short copy. Actually, it depends on your topic. If you have a promotional aim, as in the online sales letters or landing pages, a long copy could not be the right answer. Instead, if you have to introduce a product or a brand, well, it is certainly better to describe more details and write about them in a specific and longer way. Yes, long copy generally outsells short copy (in this case)!
Hence, there is not an exclusive, long and short copies work both..and the best is writing something interesting and original and creative and fulfilling and compelling…and so on.