There is no one "right size for espresso". Even with a pressurized filter basket, you can only do so much to achieve proper extraction (i.e. getting the right amount of water through the puck in the right amount of time) without controlling the grind size.

What makes an espresso cup different from the next one is not only the beans and how much water you put through it, but also what is extracted from the beans. Even a regular Joe can tell the difference between a under-extracted sour shot and something closer to proper extraction.