Just a quick tip about why you might notice some sizes are not available when you are attempting to change the size/scale level of an Azure virtual machine in the portal.
I wanted to change one of my Development VMs to a DS12_v2, but that choice wasn't available:
It didn't immediately dawn on me why it wasn't available, so I thought I'd try PowerShell:
PowerShell returned an error (shown above) that told me the specific problem. I had 9 disks attached to the existing VM, which exceeded the 8 able to be attached to the VM that I wanted. I haven't converted to using managed disks yet. So, once I reconfigured my connected storage accounts a bit, the VM size I wanted became available:
Other common reasons you might see a size unavailable:
-That machine isn't available in your selected region
-Not supported by your hardware or cluster (more info: https://azure.microsoft.com/en-us/blog/resize-virtual-machines/)
As a sidenote, if the new VM configuration adds cores which exceeds the number you are allocated, then you will have to request more cores from support. From what I've seen, support responds to these requests very quickly.