Copywriters are often asked how long promotional copy should be and the quick and easy answer is "as long as necessary." But for a more thoughtful examination, I invite you to consider the following 3 rules of thumb:
1. The more expensive your product or service, the longer your copy should be.
2. If your product or service has multiple features, you may need longer copy to adequately present them all.
3. If your product or service is something that most readers are familar with, you may be able to get your point across using shorter copy.
And more specifically...
For web sites:
Writer Nick Usborne length of web site copy should be dictated by what the site visitors expect. So when creating that copy, he suggests that you ask yourself questions like "How much do first-time visitors really want to learn from this page?" and "Have I provided enough information so they know where to click next?"
A McGraw Hill study looked at 3,597 ads in 26 different magazines. It was determined that the ads with 300 or more words were more successful than shorter ads in terms of brand awareness and prompting an action. This strongly suggests that people don't decide not to read an ad because they think it has too many words.
Your copy length should be dictated by what you want it to do for you.
If you want the copy to persuade the reader to request a free sample or additional information from you, the copy can most likely be relatively short.
However, if you're asking for more of a commitment (like asking for money or trying to close a deal), you may need to use longer copy to more fully and effectilvey get your message across.
And the direct marketer in me has to bring in the topic of "the list." People will read what they're interested in. So if you're marketing to the right list - meaning if you're marketing to people who have a need or a desire for what you have to offer - they'll read your long copy because they crave that information.
On the flip side, if you're talking to the wrong audience, they'll stop reading before the end, regardless of how many or how few words you use.