I would suggest using a fixed ratio of your preference, 4:3 matches your 2000 x 1500 screen approximation. I use the same ratio that the camera is set to, 3:2, most of the time. If there's not enough pixels left to fill your screen after cropping, you should be able to adjust your viewer to 'Fill' the screen with the image, if/when desired. My viewer/slideshow is set to 'Fit' images to my screen so that larger images are constrained to the screen, crops smaller than screen size 2880x1800 are just shown smaller than the screen, which is fine by me.