I think the key is to identify which economic and industrial sectors benefit from a degree of size, and which don't.  Some people might argue that aerospace, automotives, steel, and whatnot should be broken up, but I'm not so sure.  I think it's worth noting, though, that many big industrial companies aren't dong all that hot these days - they're not the ones draining the blood from the economy, and they're as much hurt by the pressure for insane returns as anyone.

Banks, on the other hand, seem to provide no obvious economic advantages from concentration - much the opposite.

Retail is another example.  Sure, Walmart is able to offer greater supply chain efficiencies, but this just means that more profit is sucked out of consumer pockets and concentrated into the hands of the Waltons.  An ecology of small and local retail outfits is a key part of a prosperous middle class, as small shopkeepers tend to be solid members of local communities, and employ people.  Furthermore, who really thinks that the complete and total domination of entire national markets by just a couple chains is a good thing?

Media is the final example.  I really see no good coming from nationalized media, given that there are so many people of talent and ability in all aspects of the creative fields who are desperate for employment, and so many regions and communities which are more or less ignored by the nationalized media.  Further, media concentration brings message discipline, with obvious and disastrous effects on political discourse.  Diversity is an unchallengeable good, more so in the modern era of low-cost production than ever.