This will tell you the minimum size (the size of your MDF and NDF files). The backup, unless compressed, will never be less than this.
USE {myDB};
SELECT SUM(size/128) AS mb FROM sys.database_files WHERE type = 0
"I cant stress enough the importance of switching from a sequential files mindset to set-based thinking. After you make the switch, you can spend your time tuning and optimizing your queries instead of maintaining lengthy, poor-performing code."-- Itzik Ben-Gan 2001