There is no way I know of to do that. You can only choose to download headers, or full e-mails (HIGHLY recommended in my opinion). I would also recommend that you teach users to not store too many items in one folder, since that seems to affect performance. In general, for IMAP, it is just slow with larger mailboxes.
Ultimately, if you want excellent performance with large mailboxes, and quick synchronization times, I can't recommend hosted Exchange enough. I have multiple clients using it, including a lawyer's office where they must keep everything, and it performs extremely well. Some of the users have as much as 12GB of mail, and it synchronizes header changes in seconds from a server 3000 miles away....only after that does it pull the bodies and attachments.
If you do choose to go this route, come back and I can offer some concrete advice on how to make the move, because I encountered numerous pitfalls the first time I went from IMAP to hosted Exchange, including who I would go with and why.
OK, thanks for the help. We actually use Google Apps for Business, so our users have access to Gmail, but this user just prefers Outlook. I'm going to go the "teach users to have small folders" route, because currently all of his mail is in one box- the inbox. I'll archive all of his mail and set Outlook to not sync Gmail's "All Mail" folder. – John – 2011-06-17T14:11:11.853
@John I was going to mention archiving too, but thought that was a bit obvious. – KCotreau – 2011-06-17T14:30:07.323