There's an old marketing adage, "the more you tell, the more you sell!" However, there's a caveat to that adage, "whatever you do, don't be boring!"
I personally use long copy with great success, and so do many other marketers. But it has to be skillfully done or else readers will get bored and click away.
I sell my E-mail Consultation Service for $99.95, using long copy. However, I've also made my copy interactive. While potential clients are reading my sales letter, they can also click on different links and check out my credentials, as well as read testimonials telling them what my current clients think about my service.
Here's a general rule of thumb about writing copy: Make your copy as long as necessary to effectively tell your story. Finding the correct balance between length and effectiveness requires testing, testing and more testing!