I have a 12" Macbook and an 11" Air and I think anything smaller than the Air is not really practical. I suppose if I rotate my 6s+ I could get something super tiny, then just attach a bluetooth keyboard..

But, I just don't see the point of going that small. The Macbook and Air are really light - I don't notice them much at all if I out them in my backpack (a 15" is definitely noticeable).

Anyway, it just seems like a thought exercise.