Azure SQL DB and Azure VMs

Hello all,
a my customer would like to have an OIM infrastructure on Azure Cloud but I haven't experience on it.
About the DB, I read on Installation manual that it is required a Managed Instance in Business critical tier.
Probabily other parameters are necessary to define this instance. I should use all default ? There are some particulars parameters to give attention ?

About the others components, WEB and JobServices, at the moment I agree with customer to use virtual machines.
Again there are suggestion about the Sizes for virtual machines in Azure ?  There are others some machine parameters to give attention ?

Tank you and best regards
Ermes