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.

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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/