In lieu of reading the Kimball book, this design tip discusses the Kimball approach:
http://www.kimballgroup.com/html/07dt/KU95PatternsAvoidHeader-LineItem.pdf
As Koen says, you can aggregate the case table up to the grain of you current fact table.
Or you can allocate your current fact table down to the new grain.
I wouldn't be concerned about the size of you fact table at 2m currently, unless this only covers a short timespan.
Typically, I would advise moving to the more detailed grain.
It is much easier to aggregate a detailed fact table in your reporting than to make up detail that you aren't recording.
This does depend on your requirements and whether you can realistically allocate downwards.
HTH,
Sam