Importing large dwg-files can be problematic:

When your project becomes larger than 2 miles (around 3.2km) across, Revit will run into trouble due to inaccuracies that can occur so far away from zero. In the past, all you got was a warning, but with 2010, Revit just tells you no.

But what about large masterplans that you want to use to place your project? Apart from opening them in AutoCAD, deleting everything around the site and resaving it?

Well, here's one way:

Instead of letting Revit choose the Import Units, select Custom and give it a manual number. In this case I am telling Revit that one unit in the dwg is one unit in Revit.
This is strictly speaking not true, as one unit in the dwg is one meter, whereas one unit in Revit is one mm. But hey.

I then go ahead and resize the dwg to the size I need, using the Ribbon > Modify dwg > Scale command after selecting the dwg. Even though the project is now bigger than 2 miles, I get no error.