The size of your document in can be unlimited. The blue rectangle which represents the Document Boundary exists to enable the scrollbars to work: if the document boundary was infinite you would not be able to slide the scrollbar thumb, say, halfway up the scrollbar and have a meaningful change in the view on your drawing. What, after all, is half of infinity?
The dimensions of the Document Boundary are set using the Dimensions page of the Document Properties Sheet, or it can be sized automatically using the Arrange | Document Boundary command.
The Document Boundary can be displayed or hidden using the View menu, or by typing VB