Paint.NET :: Selecting A Particular Size To Crop?

I am trying to edit a number of photos for a Ken Burns type travelogue, and that means I have to make all my color and other adjustments, and then edit to a common size (1280x720). This usually means resizing the photo which gives me the 1280, and then I need to pick which 720 of the image to include. With the rectangular select I can get close, but my mouse movement is usually too much to get exactly to 720, so I often end up having to resize again to drop/add a couple rows.

Is there some way to set a selection box to be a certain size so it could then be moved up and down in the image, and then crop the image when you have selected the portion you want?

Just to clarify, I know of the "Fixed Size" selection. But it is almost as impossible to get that lined up so that you get the full 1280 width as it is to get the bounding box dropped to exactly 720...