I like the Psion form factor, but having used a lot of laptops I think if you're traveling on a train and want to do actual work then a ten inch netbook or 14 inch laptop (like a Thinkpad) is the best kind of size for typing rather than single finger prodding. Larger than that and it starts to get unwieldy.

https://liliputing.com/2018/01/hands-gemini-pda-handheld-pc-android-linux-6-inch-display.html