[mthiim]

Many-worlds etc. does not have such a limit. There are interpretations that posit that collapse is triggered by the system reaching a certain size (also GRW theory etc.). Such interpretations would preclude big quantum computers. Note that such interpretations aren't purely interpretations because they actually do posit measurable new physical phenomena. But since the relevant experiments are not currently possible to carry out and since they do have significant interpretational consequences, they are often called intepretations.