I am rendering a small animation with Alpha that will be incorporated into a video. This animation will only occupy an area of 550 px wide by 650 px tall.
I know changing the Resolution in the Rendering tab is not the right way (as it keeps the same dimensions but with less resolution/pixels).
Maybe there’s a way to crop it? (just thinking out loud)
Thanks guys 
You can do a Border Render (recently renamed to Render Region in 2.80 to match the more common term for it) to restrict the part of the image that gets computed to a rectangle you specify.
You can enable Border Render in the Lower left corner of the Dimensions sub-panel of the Render Properties in 2.79, or Render Region in the Output Properties in 2.80. Then in a camera view you can CTRL-B and draw the rectangle you want rendered, and then an F12 render image or CTRL-F12 render animation will only compute the specific area in each frame with the rest of each frame filled with transparency, which should be exactly what you need. You can also select the associated Crop option if you want the output frames to only consist of the rendered area rather than outputting the full frame sized images.
Thank you Pixelfox.
Unfortunately Border Rendering is not supported by VSE. At least in 2.79b. I’ll try 2.80…
I fiddled around with Cropping in VSE, but doesn’t update live, and is not exactly obvious how to use it: If I enter 1000 pixels to be removed from the Left, then it keeps a large empty space to the left. If I enter 200, it deforms the image.
In 2.80 adding a Scene to VSE is still not possible, at least from the 2.80 version of a couple days ago.
It’s nice knowing blender can do this.