1. You can make a "window" template in Photoshop.
2. You can also use Titler to make a window using various black rectangles and leaving an empty space where you want the window.
There are probably many other ways, but you now have some ideas.
You maybe just looking for P in P effect. ie "Picture in Picture"
All done in the FX Window- Motion
This is an interesting problem and it does have an easy solution, but Track Matte, Titler, and PIP are not the way to go. The first two, as far as I can tell from experimenting with stills that are animated (scale changes for instance), cause the matte to also change scale, resulting in a window that changes size. That was the "offset" I spoke about, which at first appeared to me to be acting like a drop shadow. PIP's problem is that it is only available in 25%.
The solution is pretty obvious now that I am a bit more familiar with these things. What I am looking for is a window on a black background:
1. In Photoshop, generate a grayscale mask the size of the video frame using white to form the window. The rest of the mask should be black.
2. Drag this mask on to a video layer above the clip or still.
That's it. A simple black and white mask where the white is the window. The only slight problem is that you can't see the edges of the window, you can only see the edges by the effect is has on the clip underneath. That's because the white area, the window, allows you to look through to the underlying layers and you'll either see the "windowed" clip or the black of the nether regions. A colour matte placed as the lowest layer, and toggled on and off, allows the window to be seen if needed.
I would probably create a sequence that matches the size of the window you want, assuming you mean Picture In Picture.
Then just grab the finished sequence from the project panel and drop it on a 1920X1080 sequence.
The advantage of using the nested sequence is that you can pan and scan, resize to a specific part of the larger frame, or shrink the content to fit into the new, smaller sequence. Then all you have to do is drop it in. Simple.