OnTopReplica is a neat little tool that takes this functionality and lets you put one of those thumbnails anywhere on your screen, and it can be any size.

Once you run it, you select a window, and then you select a region in that window. Once that's done, anything happening in that region of the window will be shown in a large, permanent, always-on-top "thumbnail" that you can place anywhere on the screen.

How it works
Selecting the window to be thumbnailed.After installation, you can launch the application through the Start Menu (the link is called "OnTopReplica"). By right-clicking on the empty window you'll get a list of windows you can clone, an opacity setting, the option to select a subregion of the original window and finally the option to reduce the application to the taskbar.

Cloning a window: simply select the window you wish to clone from the list and you're done! You will be able to move and resize (the replicated window will always try to match the original window's aspect ratio) the window, or select another window to clone in any moment.

Select region: clicking on "Select region" will open a small side bar, allowing you to select a subregion of the original window to clone (this is particularly useful in order to trim away unnecessary GUI elements from a video player). To do so, simply click on the thumbnail and drag the region. As soon as you release the left mouse button the region is updated on the panel. You may store regions you find useful for future use as well (just click on "Save").

OnTopReplica Features:
1. Clone any of your windows and keep it always on top
2. Select a subregion of the cloned window
3. Store the selected subregions for future use
4. Auto-resizing (fit the original window, half, quarter and fullscreen mode)
5. Dock in task bar
6. Adjustable opacity

The license of this software is Free, you can free download and free use this desktop utility software.