Please test the project attached to the thread. Move to the last page of the RadGrid and you will notice that the RadDock will resize according to its content.
In case you are talking about the spacing that is present around the RadDock's content, you can remove it using the following CSS code:
.rdLeft, .rdBottom, .rdRight
With Q3.2009 we have introduced an improved rendering that solves a number of previous issues reported by our customers. This spacing is due to these changes in the dock's rendering.
the Telerik team