In my opinion, the problem here is not to tell Uncle Dave:
Dave, learn to play both size keys!

The problem is:

Why Yamaha (and not Roland, Korg, Technics) make and sell keyboards with no standard size of keys?

I know Yamaha may say: I make keyboards as I want.

Buy, then, if so, why Yamaha don't write in all its keyboards, and in a great type letter, this sentence:
ATTENTION: THIS KEYBOARD HASN'T STANDARD SIZE KEYS! ??