I have shipped fairly large clivias, with 16-20 leaves 18 inches long and lots of roots in priority mail boxes, which come in a wide variety of shapes and sizes. If you ask your post office, you might find one that's just right. You want to carefully bare root the plant and wrap it in newspaper so it's in a cylinder shape. I would be reluctant to remove or cut healthy roots because they are so necessary for the plant's survival. You can pull off a few of the outer leaves, but if you chop off the tips, it will take a long timefor the plant to get its shape back. Instead, I wrap the tip of each of the longer leaves in a piece of newspaper and gently fold it down.