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.
Number of Data Disks Allowed
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:
Cores Available in Your Azure Subscription
Another reason might be the number of cores you have available in your subscription:
By default, this is a fairly low number. If you create a support request, you can get more cores. It gets approved pretty quickly as long as it's a reasonable number.
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/)