IMAPSize Freeware. And quite powerful.
>The only workaround I could think of was to install Thunderbird on the new machine
Once you have the .EML files created on the old machine (step 3), you can move them to the new machine and complete the task there. No need to install TB on the new machine.
>or there just isn't a super easy way to do this.
I don't think there is an easy way. There are a few utilities/add-ons, but this seems to be the most straightforward method:
http://www.broobles.com/imapsize/th2outlook.php
You would need a Windows XP machine to do the import/export with Outlook Express, as Windows 7 doesn't include this app.
Or you might try installing the Windows Live Mail client on Windows 7, which might work as a substitute for Outlook Express.
If you'd like a general idea rather than message by message specifics, you could look at the following options:
Find large e-mails (3d-party tool): http://www.findbigmail.com/
Search by attachment size (in bytes): size:<bytes> so 5MB would be size:5000000
Generic IMAP utility that can delete attachments: http://www.broobles.com/imapsize/
Look for the BIG messages with BIG attachments first. Deleting a few of those can be worth many thousands of smaller messages.
Find large e-mails (3d-party tool): http://www.findbigmail.com/
Search by attachment size (in bytes): size:<bytes> so 5MB would be size:5000000
Generic IMAP utility that can delete attachments: http://www.broobles.com/imapsize/
Make sure to clear Trash (and Spam) as you don't get the space back until the messages are totally out of the account.
Some years ago Google switched from slowly adding space to Gmail accounts to adding it in chunks. Gmail originally had 1G per account and is now up to 15G, so that's better than 1G/year growth. It's been a while since the last increase, and no one knows when/if they might do it again.
Here are some tools to help you find the large messages in your account so you can be more efficient at cleanup:
Find large e-mails (3d-party tool): http://www.findbigmail.com/
Search by attachment size (in bytes): size:<bytes> so 5MB would be size:5000000
Generic IMAP utility that can delete attachments: http://www.broobles.com/imapsize/
Does this happen when you boot directly from the Win8 media? Same thing if you initiate the install from within Win7?
Here's some info. I wasn't aware that any OEM had implemented this feature:
http://www.broobles.com/imapsize/th2outlook.php
Also, you might want to post this question here:
This might be useful, I was looking for the same a while back but haven't had a chance/been brave enough to use it yet. Everything but the note, it looks like. You could make a label with all emails older than two years, and then run it maybe.