On the side bar you will see a heading called Searches.
Underneath this heading click the search called Messages Larger Than 25 MB. This will search your mail for any large messages.
The search results will appear in the message list.
When you click on the message you will see it in the preview pane. If there is one attachment, click the Download link beside it. If there is more than one attachment, click the Download All link beside them. Download them to one central location where you will be able to find them later.
Once your attachment is downloaded, click the Remove link beside it. If there was more than one attachment, click the Remove All link beside them. This will remove the attachment while retaining the message, therefore allowing it to be migrated to Google Mail.
Repeat steps 5 and 6 for all of the messages returned by the search.
When you are done, run the search again as you did in step 3, just to be sure you've handled all of the messages.