You receive an error message when you try to set the storage limits to 2 GB or larger for a mailbox or for a mailbox store in Exchange 2003 or in Exchange 2000
“The value for ‘Prohibit send and receive at (KB)’ is not valid. Specify a value from 0 through 2097151.”
If you wish to increase this size you have to use (ADSI Edit) tool.
See the following article for more information – http://support.microsoft.com/kb/930366