MSMQ controller servers (the PEC, PSCs, and BSCs) use a Microsoft SQL Server version 6.5 database to store the MQIS. You can purchase SQL Server 6.5 for each MSMQ controller server, or you can install a limited version of SQL Server 6.5 when you install each controller server.
The MSMQ limited version of SQL Server version 6.5 is limited to a cumulative total of 100 MB in database files, which leaves, at most, 87 MB for a maximum MSMQ database (with 13 MB left for the logging of MSMQ database updates). If your enterprise outgrows the limited version of SQL Server 6.5, you can purchase SQL Server 6.5, upgrade your existing MQIS installations, and increase the size of the MQIS database.