Short and fat seems like a great idea. Lower surface area to volume ratio, which should help with the efficiency. Spherical would be ideal, but also too expensive to make.

I would want one big enough so that I can set the temp just a little above comfortable shower temperature, without running out after 2 long showers (or one extra-long shower with 2 people, <img src="/images/graemlins/wink.gif" alt="" /> <img src="/images/graemlins/wink.gif" alt="" /> ). That would seem to me to be the most efficient. My washer and dishwasher both have internal heaters to bring their temperature up to what they need.

Theoretically, what would be the most energy efficient is "on demand" at each faucet. Then you don't have a tank of hot water sitting around waiting to be used, radiating energy. I understand that's how it's done in Europe in some places, but that's not the American way.