Q. What size VMs should I use for Azure Site Recovery protecting VMware VMs and physical servers for the infrastructure services?
A. When using ASR to protect VMware VMs on-premises and physical machines there is a certain amount of infrastructure in Azure, namely the configuration server and one or more master target servers.
The configuration server in Azure is typically an A3 size VM which supports up to 750 source protected volumes. This means if each VM had 3 volumes then a single A3 configuration server could support 250 source VMs.
For the master target server you can use A4 or D14 size VMs. Because the master target has to mount a disk for each protected volume that limits the number of VMs each master target can support. An A4 supports up to 16 data disks and a D14 supports 32 however in both cases one disk is reserved for retention purposes leaving an A4 supporting 15 source disks and a D14 supporting 31 source disks. Typically you would deploy many A4 VMs which is more economical than using D14s however a single VM can only be protected by a single master target which means if you have a single VM with more than 15 data disks requiring protection you would have to use a D14.
Note for the storage accounts in Azure there is a 20,000 IOPS limit for each Azure storage account however for ASR protection there is around 2.5 times the source IOPS because of the various reads, writes and caching that is performed. Therefore for each 8000 source IOPS you should create new storage accounts on the Azure side.