Reduce Size Mailbox Store Exchange 2003

Addition:
1. If OL users are in cachec mode, tell them to take PST of their mailbox.
2. Set the retention period to 0 (for deleted emails & deleted mailboxes) in properties of Mailbox Store.
3. Tell users to delete unwanted emails & delete unwanted mailboxes.
NOTE: If any email or mailbox is deleted after setting retention period to 0, it will be deleted immediately & could not be recovered unless we  have a backup.
4. Wait for online maintenance to complete & then proceed with offline defrag using below mentioned steps:
- Dismount Mailbox Store.
- Open command prompt> point directory to BIN folder (by default the location is C:\Program Files\Exchsrvr\Bin)
- After pointing to BIN directory, run command eseutil /d "Path of .edb file" (Default: C:\Program Files\Exchsrvr\mdbdata\priv1.edb)
5. Offline defrag runs at a speed of 4-5 GB/hr. So, plan maintenace accordingly.

Let me know if you have any more questions :-)

How big is quite big?  Size is different for everyone (no smutty jokes please!)

You cannot reduce you mailstore quickly if you have retention setup on it.  The default retention period is 30 days, so even if your users do a damn good job of housekeeping as Jeff has suggesed, or Auto-Archiving, nothing will happen until the 30 day period has expired and the items can be purged.

Check the settings on your store (limits tab).

To defragment the store to reduce space, you need to take the store offline.  Read the following article in case you need some guidance.

http://www.msexchange.org/tutorials/Exchange-ISINTEG-ESEUTIL.html

Exchange stores get big - it's their role. You need to have good management practice and mailbox limits to keep it down. .
To reduce the size on disk, as everyone else has mentioned use an offline defrag, but they'll only get big again.

Unless you have problems with space or backup windows I wouldn't bother with an offline defrag. The maintenance tasks that exchange runs defrag the database and that maintains their efficiency.
The unused space in the mdb file (white space) will be quickly reused

Look for an event id of 1221 in the app log to see how much space can be recovered in a offline defrag

I agree with most of the comments and advise made above, however, IMHO

1. As Mike Courtney points out MS says they will reuse the white space so dont bother with defragmentation and that is kind of true but not always, i.e. all things must be properly aligned and in short it never is so yes there are valid reasons to defragment a database and you do get allot of value from doin so, however;
2. There is allot more going on during a fragmentation then just removing white space
3. I would not recommend that you just do defragmentation on its own, i.e. you really need to know the proper use of the tools and at minimum include isinteg as part of your process as well as a pre and post backup of the information store.
4. having users cleanup ahead of time as pointed out by Jeff and others is an excellent point to ensure you get the most out to the defragmentation.
5. Rule of thumb for defragmentation is to have a minimum of 110% of the EDB size, for a work space to build the temp EDB.
6. if you run a full maintenance process, you will end up with a cleaner, faster and more stable EDB that will backup faster and God forbid restore faster if needed.