Group Policy - Event Log Service | Security | Maximum Log Size (KB) forces a minimum size of 20480 KB.
Explain text lists minimum as 1024 KB as following text:
“If you enable this policy setting, you can configure the maximum log file size to be between 1 megabyte (1024 kilobytes) and 2 terabytes (2147483647 kilobytes) in kilobyte increments.”
Eventvwr.exe lists minimum as 1024 KB
It's easy to reproduce this problem
1.    Open group policy editor
2.    Go to “administrative template” -> “Windows Components” -> “Event log service” -> “Security”
3.    Double click “maximum log size(KB)”
You can not set log size smaller than 20480KB. But explain text said yes.
We hope fix will be published soon.