What is your exchange version? If I assume Exchange 2010, the default Public Folder storage limit is 2GB. Increase the public folder storage quota.
Open Exchange management console, expand server configuration and select mailbox configuration. Right click on the Public Folder Database in the Database management pane and select Properties and limits. Here is where the limit is set to 2 GB, change this to a higher value then you can set individual values on each public folder that is higher than 2 GB.
