This option is located at: Reader -> Setup -> Settings -> Read
Viewing a large post can use up a lot of your computer’s memory, and needlessly slow things down. For example, a post that contains an 800kb picture “attachment” would take up 800kb in the Read Post window, and a lot more in Pix Player. What you see in the Read Post window is just the “encoded” image data.
There really is no such thing as an attachment. The picture is contained in the post, though it is converted into text since that is the only thing allowed in a Usenet post. NewzToolz will automatically figure out that the post contains an encoded picture, decode it, and display it in Pix Player. So, for a post that contains only the picture, there is nothing else to see in the Read Post window. However, sometimes a poster will type in some text before the picture, and you may want to read that. So, these controls are designed to permit you to see the first few lines of a post, and not waste time and memory on the encoded part of it, which you will see in Pix Player anyway.
There is no one right setting here. It mainly depends on your taste, the type of newsgroup, and the habits of the posters. For example, if you are looking at a pictures newsgroup, and don’t care what anybody types there, you can set the number of lines to a very low number and just look at the pictures in Pix Player.