Right click on your cube (in SqlServer Management Studio), and check properties.
Check StorageLocation property to see in which directory your cube is being processed.
Check your partitions' storage locations as well, just like you did with the cube (they might be in different directories).
Finally, open Windows Explorer is those directories, and sum all their sizes. This will give you actual disk space taken by that cube.
Final note: sometimes during processing, several temp files are created, and SSAS works on a duplicate of the cube, before droping it. So, check its size only after processing has been complete.