You asked for it, and now you’re going to get it. This is currently the most passionately requested feature for Paint.NET, and I’ve just finished writing it for the v3.20 release.
This will make it easy, for instance, to select a 4 inch X 6 inch area. Or to do 16×9 ratio cropping to create wallpapers for that awesome widescreen TV you just bought. I have also added a toolbar split-button for selecting the selection combine mode. This does not add new functionality to Paint.NET but does increase the discoverability of this feature, and also ensures some consistency.
The Magic Wand and Paint Bucket tools also get a toolbar button for choosing Contiguous vs. Global flood. Again, this isn’t adding new functionality but falls into the area of making an existing feature more discoverable. I haven’t found a pair of icons that I really like for this yet, so the lightning bolt and globe are currently just stand-ins: