You can always use the full size, but if the issue is that your images don’t cover enough area, you can fix that with CSS and the background-size and object-fit parameters.
This will let you use img tags that are larger than the source image, or background images smaller than their container, and allow you to tell the browser how to make the image fit, e.g. stretching it or enlarging it to cover the entire canvas while keeping proportions, etc.
Generally you want the browser to do the upscaling at runtime so that it upscales too the actual dimensions and avoids the extra bandwidth