Good article from Bryce Roberts:
This notion of a billion dollar company is steeped in silicon valley lore. … Quite often, the companies that state it as an early goal fail while those that end up being the wildest of successes, start out with a very different objective.
He goes on to show a number of good examples of companies that started with much smaller ambitions. I think he overstates it a bit as many companies that do end up with billion dollar valuations set out to do that. The key to his post, though, is that it takes time to get there. It is easy to daydream about the big goal rather than executing today.
Bottom line: a good idea is a good idea, no matter what size I think it will be.