It is worth noting the following limitations on SQL 2012 Express related to Lync Server 2013 Standard Edition and collocation of any roles:
- Limited to lesser of 1 Socket or 4 cores
- 1GB Maximum memory utilized (SQL Server Database Engine)
- 10 GB Maximum relational Database size
Then from Features Supported by the Editions of SQL Server 2012
http://msdn.microsoft.com/en-us/library/cc645993.aspx
