Paint.NET :: How To Resize Images In One Layer To Be To Scale For Image In Another Layer

I would like to make a composite image with source images making up the various layers (so I can work my magic on each layer indepent of the others). The problem I have is that not all of the images I'm using are to the right scale. What I want to do is resize those layers that are too big. Here's an example...

Let's say I have 5 pictures that I want to use to make one composite image from. The main background layer will be made from an image that is, say 800x600.

Another picture (let's call this subject 1) is 600x600 and is already at the right scale if I cut out the portion I want to keep and lay it on top of the background layer.

Yet another picture (subject 2) is huge (think 2000x2000). I would like to cut out the portion I would like to use and drop it in as a new layer and then obviously re-size it down to the correct scale. I'd prefer to do this directly in the composite image I'm making on its own layer to ensure I get the size right (and without manipulating the image guess-work style before I cut/paste it in).And so on...

I've found ways to make the canvas bigger (not what I want to do)... but not anything on how to resize the images on individual layers without impacting the other layers. Is there a way to do this?