When working on ads, I often hear comments like "There is too much copy there. Nobody will read all that".
And while this is often a valid argument, I thought it would be useful to quote Claude C Hopkins, one of the most respected advertisers ever:
"Some say 'Be very brief. People will read but little'. Would you say that to a salesman? With a prospect standing before him, would you confine him to any certain number of words? That would be an unthinkable handicap."
My own view is the amount of copy depends on the challenge. If they are online, you can say less. They can click. If it is outdoor - less is definitely more.
But in press or direct mail, as long as the copy is focused and works well with the design, long copy can work just fine. If your readers are not interested, they won't read beyond the headline, so it doesn't matter how much copy you have. But if they read the headline and are still interested, you should give them as many reasons to buy as possible.
This then becomes a question on how to balance the copy and design so they are not crowding out each other.