The emphasis of Blender development is Artistic, and not high precision modelling like is expected from dedicated CAD/Engineering software. This doesn't mean you can't model or represent big things, but it does mean that you are limited to a level of precision in one mesh.

To get an idea about precision read Ideasman42's response to [Digits of precision of vertices coordinates](http://blender.stackexchange.com/questions/3168/digits-of-precision-of-vertices-coordinates) . The technical answer to your question can be found in this response.