how can i increase the lines in email preview

increase amount of lines that display in email preview

The message preview should display the complete message. You simply need to scroll down if the message is longer than the screen. The setting Menu > View > Layout > Messages on the Right also gives more lines of text on the screen than Layout > Messages on the Bottom. If you want the messages on the bottom, you can also drag the border between messages and list to display more of the message.