No matter which type of media is delivering your message, the content needs to be as long as it needs to be to generate action from your targeted audience. That said, here are some guidelines:
- Lead generation copy is typically shorter than one-step sales or fulfillment kit copy;
- Email content with the job of generating clickthroughs to a website is generally shorter than the page(s) it links to;
- Effective email subject lines can vary in length. When in doubt, test.