Following the steps below at least yearly can make a big difference.  If you can find time to do it 2-3 times each school year that would be even better.  A video of these steps is available here.

1. Start GroupWise and log in.
2. Choose Tools->Find from the menu (or press Ctrl+F)
   
3. Check the box next to "Mail" for the item type then click "Advanced Find..."
   
4. In the advanced find window, click the button that says "End"
   
5. Select "And" from the pop-up menu
   
6. In the new row that appears, select "Size" from the pop-up
   (If "Size" is not an option then select "All Fields..." to find it.)
   
7. Click the button with "=" on it and choose ">= Greater Than or Equal To" from the pop-up
   
8. Type the size (in bytes) of the mail you want to search for.  (Two million is usually a pretty good choice.)  You should end up with something like this.  Then click OK
   
9. Uncheck "All Libraries" on the right side and then click OK
   
10. You should end up with something like this:
    
11. Now you can go through these large emails and delete ones you no longer need.  (Don't forget to empty your trash afterwards.)
12. If you want to see the actual message sizes and optionally sort by size continue with the steps below.
13. Right-Click in on the "To" column heading in your Find Results and select "More columns" from the pop-up menu.
    
14.  In the windows that appears, scroll down and select "Size" in the left column and click the ">> Add >>" button to move it to the right column.
    
15. Scroll down and select "Size" in the right column now and click the "Up" button several times to move it up near the top of the list.  Then click OK.
    
16. Finally, if you want to sort by size just click the "Size" column heading.