As companies adopt Outlook 2003 cached mode for their users, the Offline Address Book (OAB) becomes increasingly important. When Outlook 2003 users open the GAL or use name resolution, the OAB is used instead of making requests to a Global Catalog server.
The OAB itself consists of a set of .OAB files which are attached to a message object and stored in a special public folder (commonly called a system folder). The size of these files will vary depending on how many mail-enabled users you have in the Active Directory and how many attributes are present on each object. In my travels, I find that a good back-of-the-envelope guide is 1MB per 1000 users.
The OAB is typically generated once a day. OABGen, which is part of the Exchange System Attendant process (Management and Administration Daemon or MAD) runs on one nominated Exchange server. First of all it looks at the previous day's OAB, and creates a new message object with an attached changes.oab file. This includes all the changes that were made in the past 24 hours, and is the mechanism that Outlook uses to download incremental updates. Next, OABGen will refresh the main .OAB files which is a complete copy of the offline address book.
The OAB system folder (called OAB Version 3a in Exchange 2003) has a default aging limit of 30 days. This means that the Exchange server retains 30 days worth of changed objects until it starts automatically removing old generations. This means that Outlook users can download incremental OAB updates at up to 30 day intervals. If you download the OAB today, then wait 35 days and attempt to download an incremental update, you won't be able to; and Outlook will automatically download the full OAB again.
If you want to look at the size of your OAB files, you can use a little-known trick in ESM from Exchange 2003. Drill down to the Public Folders node, then right-click, and choose View System Folders. Once here, you can find the OAB Version 3a folder, and in the tabs on the right-hand side, choose Content. You should be able to see the last 30 days worth of changes.oab and one larger object containing the complete set of OAB files. The "inline OWA" within ESM will also tell you the file sizes of each attachment.
Finally, a word of warning. There are some instances where Outlook will need to download the complete OAB, even though you expect only the changes to be downloaded. The first scenario is where more than 1/8th of the GAL has been modified in the past 24 hours. The second scenario is when you've added a new Administrative Group to your topology. The latter can be an unexpected 'gotcha' if you don't know about it.