While you might create long sales letters for your products, shorter copy is ok for your squeeze page. (Half a page to one page of text.) Make sure that you have a compelling headline that grabs attention, as well as bullet points that point out the benefits of joining your list.
Make your opt-in box visible above the fold. You might even consider putting the box both above the fold and below the fold to make sure people can easily sign up. My preference is short copy with opt-in box at the bottom, but all “above the fold.”
There are some niches that do well with graphics and other added features on the squeeze page. There are others that do better with very simple, no-frills design. The best thing for you to do is to split test different designs to see which increases your opt-in rates. Aweber and can help you figure this out with their split-testing features.
Thrive Leads is perfect for setting up your squeeze pages – they have a bunch of templates you can use (and modify).
You may be surprised at how much a simple change (like a change in color) can increase or decrease opt-ins. While testing might be time consuming, you'll get results that will add money to your pocket.