Go to Neighbor Post Preview Options page in WordPress admin area (Options -> Neighbor Post Preview) and you’ll see 3 options to customize:
- Excerpt length. How many words do you want to display when there is not an explicit excerpt?
- Delay before hide. How long will the preview stay after your mouse leaves the link?
- CSS class name of preview window. You can customize the look of the preview window.
- Show post title in preview window. Many users just use "previous post/next post" as navigation link text, and this option will be very useful for them.