How to size SQL Server VM so that it is comparable to bare metal primary SQL server?
This is the online Microsoft guide about it:
https://docs.microsoft.com/en-us/azure/azure-sql/virtual-machines/windows/performance-guidelines-best-practices
interesting to point out:

If you are creating a new SQL Server on Azure VM and are not migrating
a current source system, create your new SQL Server VM based on your
vendor requirements. The vendor requirements for a SQL Server VM are
the same as what you would deploy on-premises.

Not to mouch after all. Said that I believe you have to do some test, but how?
You have to capture the on premise workload and reply it to an the azure vm and check the results.
A goo tool for doing it is this:
https://github.com/spaghettidba/WorkloadTools/wiki