How much resource is each SQL database using?. There are different types of resource on the database server. Memory , disk,  IOPS, CPU.

The main difficulty with defining the resource usage for a specific database is the wide impact of one query across multiple databases. For example a query with a SORT may spill into the TempDB.

Other examples , are joins and transactions across more than one database

 These links to post showing techniques to isolate different resource usage. Analysing the resource usage from different angles , should give you a rough idea of usage .  Use this information to make decisions such as – add another database ? or move a database ? or add more resource to the server.

 If you’re  tuning – tuning at the database level can be frustrating – which is why it’s more effective at the query level .

 Memory 

 SQL Server – Calculate Database memory usage with sys.dm_os_buffer_descriptors  

Database Size – extra disk space

SQL Server Database Size and Free Disk Space ... - SQL Server

IOPS 

Measure current read & writes . Do the server drives have capacity?

 SQL Server measure i\o transaction rates and sys.dm_io_virtual_file_stats

CPU Utilization

 This one is difficult – I’ve achieved the best results  using the DMV - sys.dm_exec_query_stats . Read more on SQL Server – Find high impact queries with sys.dm_exec_query_stats