In my experience 16” is definitely too big for economy seats. They sell 13” oled portable monitors that I’m very curious about, but have yet to try. That’s probably as large as I would personally go for in-flight entertainment. I’m slightly farsighted, so I usually prefer smaller screens that take up less of my fov. YMMV. Personally, the steam deck’s screen has been more than ample for me and my strange eyes when on a flight. I’ve even used my steam deck to do work with it resting on a table with a kickstand. I’ve never felt the need to use a portable monitor with it unless I needed multi monitor.