You have to use a Big View Notification Style but it is only available in 4.1 or newer
quote from here:
A notification's big view appears only when the notification is expanded, which happens when the notification is at the top of the notification drawer, or when the user expands the notification with a gesture. Expanded notifications are available starting with Android 4.1.
in the same document, you can find a sample how to use this. there are also som tips/hints how to maintain compatibility with older devices (not only regarded to the big view)