Database size can vary greatly depending on what you're scanning and how much history you're keeping. We generally recommend reserving 1GB per 1000 assets. For 1000+ assets we do recommend using SQL Server, not SQL Compact, even if you won't reach SQL Compact's database size limit. SQL Compact can't handle large amounts of assets very well.
SQL Server 2008 R2, 2012 and 2014 Express have a database size limit of 10GB, whereas older SQL Express versions have a database size limit of 4GB.