How do I save at cropped resolution?

Crop as you desire. 

Export as DNG. Import that at the same time if desired. 

Now the crop IS parametric; it's non destructive. You'll see the crop upon import but all the image data still exists and you can uncrop at any time. There's a feature like this in Photoshop as well in terms of keeping the pixels as an option after a crop. That isn't an option for Lightroom Classic with raw data. You can crop, but you retain all the pixels outside the crop. 

It isn't possible to save the DNG file with only the cropped pixels. It will be necessary to save the cropped image in a different format such as a PSD or TIF or JPEG image to have only the cropped image.

As far as the size of the DNG file is concerned, that size comparison will vary depending on the manufacturer of the original file.