What is the biggest physical thing in all existence? The universe. The universe is a limited amount of space. Even if the universe were to keep expanding forever it would only take up a limited amount of space. So if the rock was the size of the entire universe an all-powerful God could still lift it.
The only option is for God to create something that exceeds his power to lift it. But Gods power is infinite so the rock would have to be infinite in size.
Here is where we run in to a big problem. A rock is a finite object. In other words it is a limited thing. We would be asking God to take something that is finite by nature and make it infinite in size. This is both logically and actually impossible.