For me, it's not so much the screen size as the ergonomics. Haviong the screen at the correct height to enable good posture. Separate mouse an keyboard for comfort.
Then you need an external keyboard, mouse and mousepad. At that point you're not far off from just slapping a monitor on the table instead of a laptop stand.
You can get a 15-16" OLED external display with a battery for around 300 western monetary units. Half that if you stick to IPS panels[0]. You can shove one of those in a backpack easily.
[0] https://www.amazon.de/-/en/Portable-Monitor-Connection-Raspb...