Thanks for the prompt response Joe. Your suggestion is appreciated, and I believe that it is exactly what we've done, however the result isn't successful. I tried attaching screenshots of the steps we took but my account doesn't have permissions to use hyperlinks in posts. If there's another way to send an image, please let me know.
1. Choose All Domains, then "Change all user's settings in the selected domain(s)", and "Change new user defaults for the selected domain(s)"
2. Select Mailbox Size Limit (8000), then click Propagate Now