Yes, I'd say it is possible to be too big. But it really depends on what sort of meaningful interaction with other characters (or cities, or planets, etc) you want to be able to do. And, of course, given the cyclical nature of reality, once you pass a certain degree of 'big' you become 'small' (or reality shifts to a different scale around you).
Sometimes I wrestle with my demons... and sometimes we just snuggle.