What is the maximize size of an Access database and what options do I have if I reach this limit? (eJukebox current DB format)
The maximum size of an Access database is two (2) gigabytes.

For Access databases that have reached this limit, you have a number of options.

One option would be to split the frontend database (forms and reports) from the backend database (tables and queries). This is called a split database. You can then link more than one backend database to the front end database with each backend database having a size limit of 2 gigabytes.

A second option when the size limit has been reached is upsize the backend database (tables and quieries) to SQL Server. This is called upsizing the database. There is a free version of SQL Server that you can use (the version depends on your version of Access) which has a size limitation of between 4 GB and 10 GB. SQL Server 2005 Express has a size limitation of 4 GB while SQL Server 2008 R2 Express and SQL Server 2012 Express have a size limitation of 10 GB.

There are tools available for facilitating the transfer of tables and data to SQL Server. These tools greatly reduce the amount of time necessary for migrating from an Access database to a SQL Server database. Because SQL Server does not have any direct form building capabilities you will still need Microsoft Access or another form building platform (Microsoft .NET Windows forms, ASP.NET web forms, etc.) to display and edit the SQL Server data.

SQLite for Windows Runtime Free
SQLite is a software library that implements a self-contained, serverless, zero-configuration, transactional SQL database engine. Maybe this would work they use it in MediaMonkey get over 400000 songs