The Raspberry Pi circuit board is a 85mm x 56mm rectangle although this does not include the connectors that stick out from the circuit board. It is 17mm high and weighs 45g.
The following diagram has been made available by Gert, one of the engineers working with the Raspberry Pi Foundation. He took a beta-board and made measurements manually. We will have to wait and see if the dimensions are the same on the production boards.
Hopefully this diagram will be updated to accurately reflect the production boards as this information is useful for anyone designing their own cases or enclosures.