Well, everything is explained there. As you can add custom apps to the list of those ones that can be used Launchy, you can define a list of extensions usually associated with downloads, and use as the app the one linked there. So, when you encounter a file you want to download, you select Get File Size in the Launchy entry integrated in the context menu in Firefox, and theoretically the program will give you the details on the download and allow you to get the file from the program. In theory :)