…is 14”. It is the perfect balance between portability, weight, and screen real estate. I’ve tried 13”, 14”, 15”, and 16”. 14” is where it’s at.
Having used a few different sizes, I agree, for portability 14" is perfect, but I would add 1 thing. The screen should be 16:10, not 16:9. I have a Thinkpad T61 and its screen size is 16x10, to me that is the perfect Laptop. One more thing, no number pad. It should have a keybord like the old thinkpads.
It has no fan, but it rarely even gets warm, with no noticeable throttling.
I used an MB or MBP for the entirety of the Intel era, but these days I have a really hard time seeing any meaningful benefit of its increased size and bulk over the MBA.