RE: Where can I find the size of the database?
Dan,
With SO Administrator that came with RMS connect to your database, and select "database button" from menu bar then select "Properties". This will give you all sorts of information including the size and how full is it. SQL Express 2005 quota is 2000Mb where in SQL 2008 Express is 4000Mb.
If it reaches capacity the common error is that it will not record your receipts and transactions may be not complete. in that case deleting old journal entries and reindexing database can give space. But ... always before you start anything backup your database first.