It’s a matter of preference.
See, given that the machine is fully upgradable, to be quite frank, 17 inch is an ideal size for the components, and for the customization.
For how thin the laptop can be, I’d reference the dimensions of the Lenovo Legion 5i 17. The Framework 17 would thus, have bigger fans, bigger vents, and potentially, greater battery size.

Somebody give the company millions of dollars so that they increase their production capacity, design a 17 inch version of the laptop, improve the durability, and lower the cost of the products so that even people from third world countries, with some effort, can afford some for themselves and also support the Framework company in the long run.

They’re doing an amazing job quality wise.