I am not sure how to pull this off in practice but I think it would be healthy for an economy if companies couldn't grow beyond a certain size. Once companies get too big they pretty much kill new and innovative companies. Just look at how quickly small companies in the 70s and early 80s innovated around personal computing and how little innovation is happening in the smart phone or search engine area now because Google and Apple are pretty much trampling over the small guys.

Again, I have no idea how to do this in practice.