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 . The technical answer to your question can be found in this response.
Another resource which can really give a sense of the relative scales we're talking about is Alex Januszkiewicz's Article On AutoCad Accuracy. An excerpt:
If you try to do a drawing that extends 4 km in each direction, Microstation's 32 bit format will limit your accuracy to 0.001 mm. The same size AutoCAD drawing can be drawn with an accuracy of 0.000000001 mm, that is one million times more accurate.
This refers to 32 bit Single Precision (Blender uses this) vs AutoCAD's 64 bit Double Precision storage.
The link for that text may not be available permanently, but if you google "Alex Januszkiewicz AutoCAD accuracy" you will find it.