Let's get physical(units)

Very interesting points raised.

Physical size matters when you need to touch an interface (must be in the same scale as the finger) but then it becomes irrelevant if you want to display it on a big screen or projector. Fixing this with angular units (depends on viewing distance too) seems quite difficult to me, given the many subtelties of the human brain regarding distance and perceived sizes.

Also, viewing distance would change and you would need to standardize arm's length.

Or maybe not, then you may need to take the measurements of the user before using the interface. A long armed user with thick finger would then be in a delicate position, with icons too small for fingers.

But I digress...