When Giuseppe asked me this question about a year ago, I did not have the proper cups and balls in stock to test with.
After doing some tests, I can safely say that any ball smaller than 1" will not perform the penetration effect that Giuseppe wants to do.
A 1 1/8" ball works every time - those are the recommended balls for the Mendoza Combo cups.
A 1" ball will work, but you need to orient the ball properly to ensure a good pick up. Or, if you can "swirl" the ball when it is between the cups you get a good pick up regardless of orientation. If you can swirl the ball when it is between the cups, even a properly adjusted 7/8" ball will work, but it can be intermittent and the act of moving the stack to swirl the ball may distract from the of the effect - but that is personal preference. IMO, if I'm doing this effect with 1" balls, I'm swirling that ball every time to be sure that the ball vanishes.

In all cases, the balls should be adjusted.

It's really a balancing act.

Thank You to all Café Members for the contributions to this thread. I want to thank Mr. Bill Palmer who was so kind to PM me about the subject (to the point observations, Mr. Palmer, I greatly appreciated). A special thanks to DDecae for taking time and care to personally test the Mendoza Cups. Time ago, I experimented the very same moves (swirling actions and so on) and I second Your thoughts. Finally, the move I spoke about at the beginning of this thread is accomplished easly and smoothly with the Johnson (combo) cups and the 3/42 balls, the same Mr. Mendoza uses on his dvd series. I grab a chance to say thank You to Mr. Mendoza as well, a performer I admire very much and a nice and kind person.
Also forgive this late reply: I had a tough...relocation! Good Easter to all Café Members!