You may need to find the GUID's representing your Exchange 2003 mailbox databases. For example say you want to increase the database size limit to 75GB which is a registry change. This requires you to know the mailbox GUID's as shown in the image below:
An Exchange Engineer named "Bharat Suneja" wrote a really good visual basic script for achieving this which can be downloaded from here:
This script requires the Microsoft ArrayConvert libaries to be installed to function correctly. To download the ArrayConvert libaries visit Microsoft KB250344:
When downloaded extract the ArrayConvert files then copy and past all extracted files to the servers System32 directory:
Register the ADs.dll file using "regsvr32.exe ADs.dll" from the command prompt.
Navigate to the folder containing Bharat Suneja's script in command prompt and run the script using the cscript utility.