How can I get physical disk size of all rows that include an XML column?

Question

Hi all,
  I'm trying to do some analysis of our current system to evaluate our scaling needs in the future.  I would like to create a query that will sum the physical disk usage for each row, then return the Id of the row and the corresponding size on disk.  I've tried using the report server, but it just doesn't give me the detail I need.  Any help would be greatly appreciated. I've found several posts on finding the maximum possible row size based on the columns, but this always returns -1 in the XML column.

Thanks,
Todd