Hi,
As far as I know, there is no maximum size for event Log in Windows Server 2008 and Windows Server 2008 R2.
If you set a very large value in Windows Server 2008 and Windows Server 2008 R2, it will say the maximum log size could be set is 18014398509481983KB. 18014398509481983KB is 17179869184GB and the event logs can grow to whatever disk space is available.
Although the event Log size is unlimited, the recommended maximum size for each log is 4GB. Setting the size lager than 4GB may cause potential issues.
For additional information about recommended settings for event log sizes, please refer to the following article:
Recommended settings for event log sizes in Windows Server 2003 and in Windows Server 2008
http://support.microsoft.com/kb/957662
Hope it helps.
Tim Quan - MSFT