By default, the iPad email app allows you to preview just two lines of the message. I want more. Here's how I got it.
First Navigate to Settings >> Mail, Contacts, Calendars >> Preview
In the Preview section, decide the number of email lines you want viewable. I usually select five lines.
Close out of Settings. Open Mail. There you go. Now you have a bigger, better email message preview.
Author note: This tip also works with the iPhone and iPod touch.