I tried to search in regards to this query but couldn't find anything on this specifically. I have noticed when working with Paint.NET that when you try to resize a large selection down you incur a noticable loss of quality, the larger the disparity of the original and new size the more noticable the loss in quality - this only appears to happen when you downsize a selection, and not when you use the Image -> Resize option from the menu bar.
The workaround I've been using is to first use the menu bar resize to slightly bigger than I need the selection and then making precise adjustments directly. I can sort of understand why this would happen, as the on-the-fly processing of a selection resize must be using a low quality setting for speed purposes - but isn't it possible to program selection resize in such a way that you can get the selection to the size you want in this on-the-fly manner and then when you finish resizing have the program re-calculate the new size for best quality? This problem caused me no end of hassle when I first started using Paint.NET, mostly because I was becoming aware of high quality images that had taken significant losses in quality and was stumped to what was causing it until I figured it out - maybe Paint.NET, if incapable of doing the above, should present a warning to the user when resizing a very large selection down to a much smaller size that this will result in a loss of quality and that the menu resize should be utilised first?