The "preview window" was a feature added to the software in v7.2. You will need at least that version to see the specific featured mentioned here.

The size of the preview window is determined by css within the system. Note also that with the use of the RWD design created in v7.3 the width of this window can be drastically affected. If your window with is smaller than one of the breakpoints in the RWD css you may see the "mobile view" of this popup so take that into consideration when setting the width of this window. If it's set small enough a mobile view may be prevalent.

Look for the following css within your custom template sets theme_styles.css file:

And note your css could vary slightly from that below depending on changes from version to version.

iframe.listing_preview {
width: 800px;
height: 500px;
overflow: auto;
margin: 15px;
border: 2px dashed #777;
border-radius: 15px;
}

You can find the theme_styles.css file in the following path of your custom template set:

/external/css/

You can adjust the above "width" and "height" dimensions to size up or down the preview window. Note that the "position" of the preview window on the screen is determined by javascript at the time the preview window is displayed in the page.

If you don't see the above css within your custom template sets theme_styles.css you'll need to put it there so you can adjust it. You'll need to copy the above into your custom theme_styles.css or copy that from the theme_styles.css of your default template set. If you have to copy it in you can place in anywhere but at the end would make it easier to find. Once in your custom template sets theme_styles.css file make your changes and save. Then restart your place a listing process to see the results.