If you want to create images that have the same aspect ratio then what you do is you set one dimension to a value and the other to 0 when creating adding an image size
add_image_size('height1000', 0, 1000, false);
add_image_size('height800', 0, 800, false);
add_image_size('height600', 0, 600, false);
The same can be done for width
