@Clive Wightman Thanks for posting your question in the Microsoft Q&A forum.
To create a report that shows the size of the databases and the percentage of space used, you can use Azure Monitor. Azure Monitor provides metrics and logs that you can use to monitor the performance and health of your Azure resources, including Azure SQL Managed Instance.
If you are looking for instance level it's in the portal (max used vs max reserved size - 2 counters you can add) under metrics. DB level would be a T-SQL query that looks at the file size vs. the space. There is also a DMV that contains the reserved storage space and used space (instance level) if you want to keep it all in T-SQL - master.sys.server_resource_stats
Regards
Geetha