Sourceset selecting by width, slider settings specify height

If the slider settings specify a height for mobile, where do we specify the image sizes to be saved? In what functions file, or how? We have a very wide image, 1900 x 450, and have set the desktop height at 450. I have set mobile height at 300 but I do not see wide images at that height on the server in the uploads folder. Srcset is pulling the images available from the regular media sizes as well as a 1536 wide and a 768 wide, all of which are too short in height and stretching them to fit the height. We want to lose the edges and show a much crisper middle of the image on narrower devices.

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