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

http://support.microsoft.com/kb/930366
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

ADUC can only do 2GB

you use  adsiedit.msc or admodify to change to higher limit

or tick off limits, and let it grow ...