what is the maximum capacity allowed for SQL managed Instance?
At this stage, IaaS is the only option for exceeding this size. Currently, Managed Instances carry the 4TB/8TB limit depending on your service tier and I don't believe there are any announcements about increasing this. Azure SQL Database can be sized up to 4TB, again depending on the service tier chosen.

There is a new tier for Azure SQL Database in preview called Hyperscale (https://docs.microsoft.com/en-us/azure/sql-database/sql-database-service-tier-hyperscale) which may provide a solution, but this is not generally available yet. This only appears to apply for Azure SQL Database, and not Managed Instances according to the documentation available so wouldn't be an exact answer for the Managed Instance limits.