I understand you wish to fetch these values through REST API or Python SDK. The disallowed values for Admin Username can be found here:
Disallowed values: "administrator", "admin", "user", "user1", "test", "user2", "test1", "user3", "admin1", "1", "123", "a", "actuser", "adm", "admin2", "aspnet", "backup", "console", "david", "guest", "john", "owner", "root", "server", "sql", "support", "support_388945a0", "sys", "test2", "test3", "user4", "user5".
These values are not fetchable via Python or REST API. If needed you could code these values into script/code.
For "minimum and maximum values of azure disk size" this depends on the disk type. You cannot shrink a disk, so the current size would typically be the minimum size, for new disks the minimum is 4 GiB. The maximum size for OS disks 4,095 GiB, though by default the MBR limits this to 2 TiB. Again, this is not something that can be fetched. If you wish to enforce these limits, you will need to set them in your code. More information on this page.
Hope this helps! Let me know if you still have question.
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