First put Outlook express in offline mode.

In Outlook express then  create blank folders with the names you want to give to the original folders.
E.g.:  Inbox-new, Sent-new

Do not create them as subfolders of inbox as those defeats the purpose. They should be on the same level as the Inbox.
Click on the newly created folders so that Outlook express will create blank files with those names.

Note sure if the above is necessary but I had to do it.
Make sure you make a note of their location & or the location of the inbox.
You can find this location by right clicking on the inbox & copying the path
Close Outlook express.

Navigate to the folder where Outlook Express stores the dbx files. 
In that folder  you should see the above newly created files,  
They are typically 75kb in size.

Now delete or move out these new files created by outlook express & rename the original files to the new names you gave.
Inbox.dbx TO Inbox-new.dbx
Sent Items.dbx TO Sent-new.dbx

When you next open up Outlook Express, it will create new default folders for any default folders that do not exist.
Default folders are Inbox, deleted items, Sent Items & Outbox.

The original folders whose files you renamed, will now  be available in the new names you created.

Note:  The same applies to subfolders. However if you rename a folder & you have message rules that route emails to this folder , the message rule follows the folder. I.e: the email is now routed to foldername-new.  which is over the 2Gb size & hence you will continue to recieve the error message. Make sure that you manually modify the rule to route the email to foldername.