It is not possible to change the size of a VLX preview window in YIPI because the preview image provided by the VLX is a fixed size. However, you can stretch the image to a larger size using some custom HTML.
To do this, you need to create a separate HTML file and load it to the QX. In this file, you will define the size of the image. Then, you add an iframe element in YIPI (not a VLX preview window) with a link to the page you created.
Below is a screenshot of a larger custom preview window. As you can see, since we can only stretch the image, it is pretty distorted. But this technique will allow you to create a preview window of a different size to fit your interface design.
Editing the HTML file is simple - all you need to do is open it in a text editor and modify the IP address to match your VLX IP address. You will also need to modify the image width. See line 6 in the screenshot below:
Once edited and saved, you will need to FTP this file to the 'wwwpub' on your QX controller.
In YIPI, you will need to add an iframe, resize it to the desired size, then set the URL to http://[your-qx-ip-address]/custom-vlx-preview.html
Save and publish your YIPI file, and preview your file in a web browser. To resize multiple preview windows, you will need to create a new HTML file for each with a unique name.
Files are posted below, remove the ".txt" extension.