Before you reduce the size of your VM's OS disk

Even the default size for an OS disk is 127 GB and Azure does not let you resize to a smaller size, there is a way to work around this limitation with some effort from your part.

You can reduce cloud costs by using our app to power on/off Azure VMs, but if you want to go even further you can...

The procedure

Steps to reduce the size of an OS disk are well documented by Mike Lapierre https://blogs.msdn.microsoft.com/premier_developer/2018/01/26/how-to-shrink-a-managed-disk/